Energy harvesting from current loops

ABSTRACT

A system includes a two-conductor loop in which the loop current or current signal is controlled by a loop current controller to be proportional to a signal output from a sensor. The system further includes energy harvesting circuitry in electrical connection with the two-conductor loop which includes a second current controller in parallel electrical connection with the loop current controller and a power converter in electrical connection with the second current controller. The second current controls a portion of current drawn from the two-conductor loop and delivered to the power converter from an output port thereof. The portion of the current drawn from the two-conductor loop is returned to the loop current controller from the energy harvesting circuit. Noise in the portion of the current drawn from the two-conductor loop by the second current controller is controlled by the second current controller to be below a predetermined threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 62/702,012, filed Jul. 23, 2018, the disclosure of which isincorporated herein by reference.

BACKGROUND

The following information is provided to assist the reader inunderstanding technologies disclosed below and the environment in whichsuch technologies may typically be used. The terms used herein are notintended to be limited to any particular narrow interpretation unlessclearly stated otherwise in this document. References set forth hereinmay facilitate understanding of the technologies or the backgroundthereof. The disclosure of all references cited herein are incorporatedby reference.

Current loops have been used to transmit data for process monitoring andcontrol since the 1950s. As a result of low implementation cost,inherent resistance to noise, and ability to carry signals longdistances, current loops have proven particularly well-suited forindustrial environments.

Current loops include four common components: a current controller ortransmitter that modulates the current in the loop to communicateinformation or control; a receiver that measures the loop current andinterprets the information represented by the current; a power supplythat provides the voltage required to drive the required range ofcurrent around the loop and a wire that connects the transmitter andreceiver (and sometimes the power supply) in series to create a confinedconductive path for the current flow. Current loops are classified bythe manner in which these components are connected. In all instances,the transmitter, receiver and connecting wire carrying the current beingmodulated by the transmitter are in series. As illustrated in FIG. 1A,current loops are classified by the manner in which the power supply isconnected to the system. In 3- and 4-wire transmitter embodiments, thepower available to the transmitter is not affected directly by the loopcurrent because of the independent connections to the supply. In the2-wire transmitter system, the transmitter, receiver load, loop wireresistance and power supply are all in series with the current loop.Because of the external voltage supply placement in series with thecurrent loop, the power available to the 2-wire transmitter is impactednot only by the external supply voltage but also by the loop current andthe voltage drops of series loads (such as the receiver and wireresistances) in the loop, which are also dependent on the loop current.The magnitude and range of the loop current within which the transmitteris required to communicate, the external power supply and line lossesaround the loop thus represent important considerations impacting thepower available to the 2-wire transmitter.

The dominant standard for current loop communications is the 4 to 20milliamp (mA) protocol. In that protocol, information is passed betweentransmitter and receiver as a fraction of a predefined full scale rangeof current, wherein 4 mA represents the minimum value and 20 mArepresents the maximum value. A total of 16 mA represents the full scalerange. Loop currents below 4 mA are commonly employed to signal faultconditions detected at the transmitter. Currents above 20 mA arecommonly used to indicate other abnormal conditions such as overrange.In typical applications, the required working current range may includea continuum of magnitudes at or less than 3 mA and extending as high as22 mA.

Referring to FIG. 1B, the 2-wire transmitter must acquire power to bothregulate the loop current and supply circuits within or connected to thetransmitter from the power (Pport) available at the 4-20 mA port.Sometimes, all elements connected to the 4-20 mA port (other than theexternal loop components), including the loop control circuit and anycomponents/loads connected thereto are referred to herein collectivelyas the transmitter. Pport is limited by the magnitude of ILoop and theport voltage (Vport) remaining from the external power supply lessvoltage losses incurred by the wire, receiver resistance (Rrec) andother loads connected to the line. Expressed simply: Pport=Vport*ILoopwhere Vport=Vsupply−ILoop*(Rwire+Rrec) any other loop losses. While themagnitude and range of ILoop is defined by the 4-20 mA communicationsprotocol, Vport, (and hence, Pport) are influenced and limited by theexternal loop components. In practice, these external loop componentsare often part of a preexisting installation and commonly assumestandard values adopted by industry. For example, the loop supplyvoltage (Vsupply), which determines the maximum voltage available toVport, is typically fixed at a standard voltage (most frequently 24volts). While it is desirable to minimize the receiver resistance Rrecto limit the voltage drop at the receiver, this resistance is oftenselected so that the voltage drop at ILoop=20 mA rises to 10 volts(Rrec=500 ohms) or 5 volts (Rrec=250 ohms). Alternatively, thatresistance is selected to convert the resolution of ILoop into a voltagesteps resolvable by the receiver. The 2-wire transmitter is used mostfrequently in remote applications with preexisting wiring or in newinstallations where the cost of purchasing and installing a 3^(rd) or4^(th) wire over long distances is prohibitive. To minimize expense andcomplexity, long continuous lengths of wire are typically preferred toemploying repeaters. Furthermore, minimal gauge wire, with higherresistivity, is often selected to save cost. As a result, the loop wireresistance (Rwire) often represents the largest resistive load in theloop. To accommodate the broadest range of loop component combinations,it is desirable that the transmitter be capable of adapting to thebroadest range of available port power delivered by the external loop inacquiring power needed for internal operation and attached peripherals.To allocate a maximum margin for voltage losses of Rwire and otherexternal loads (to facilitate the longest wire loop lengths possible),it is advantageous that the 2-wire transmitter be capable of acquiringsufficient power at the lowest possible Vport voltages across therequired range of ILoop.

In practice, the power available to supply internal transmitter circuitsis less than the available port power defined by Vport and ILoop. Someof the common mechanisms accounting for this reduced internal poweravailability are depicted in the block diagram of a typical legacytransmitter in FIG. 1B. ILoop is typically routed through lossy circuitssuch as protection devices at the port and resistive loop currentmeasurement circuits, resulting in voltage losses reducing Vport to Vinalong this path. While it is possible to connect ILoop in series withthe internal transmitter loads, it is a more common practice to split itinto two constituents: a current directed to the transmitter internalcircuits (I2) and a shunt current (I1) that is modulated by a currentcontroller to maintain ILoop=I1+I2 at a setpoint prescribed by an analogcircuit or digital-to-analog converter (DAC) representing a processmeasurement or control scaled to 4-20 mA. In such a configuration, themaximum available internal power Pin_(Available) is approximatelydefined by the product of I2 and Vin. To maximize Pin_(Available), it isgenerally preferable to allocate as much of ILoop as possible to I2 bylimiting I1. However, the maximum allocation to I2 (I2_(max)) must berestricted to ensure adequate overhead for I1 to sink current toregulate ILoop and to support implementation of digital signalingprotocols such as wired HART™ which is most conveniently implementedthrough modulation of I1. Depending on the source of feedback for theclosed loop control driving I1, I2 is either recombined with I1 at theport terminal (in the case where feedback for ILoop control is obtaineddirectly from the loop current measurement circuit) or recombined withI1 at a summing resistor with I1 controlled such thatI1=ILoop_(setpoint)−I2, where ILoop_(setpoint) is the target controlsetpoint for ILoop. In the summing resistor arrangement, an additionalvoltage loss occurs as ILoop passes through Rsum effectively raising thetransmitter local ground (local_GND) voltage above the Vport negativeterminal and further increasing the voltage loss from Vport to Vin.

Because Vin varies with Vport and ILoop, it is not well suited as adirect supply for transmitter loads requiring a fixed voltage.Transmitters thus typically employ a linear voltage controller toregulate the variable Vin to an output voltage (Vreg_Out) suitable forsupplying such loads. Because this regulator requires overhead(V_(dropout)) to maintain the fixed output voltage, Vreg_Out isrestricted to a prescribed minimum Vin voltage (Vin_(min)) so that themaximum Vreg_Out is limited to Vreg_Outmax≤Vin_(min)−V_(dropout). Thepower available to the regulated transmitter loads is thus restricted toPload_(Available)=ILoad*Vreg_Out. The residual difference in powerPin_(Available)−Pload_(Available) is dissipated as heat in theregulator. Vin_(min) further establishes a fundamental minimum limitthat Vport must meet or exceed at the highest value for ILoop, impactingthe transmitter capability to accommodate the high loop resistances(most notably limiting the loop length as a result of the cumulativewire resistance with increased loop distance).

While the 2-wire transmitter must acquire power from the loop, itsprimary purpose is to modulate and control ILoop to communicateinformation or control. The number of discrete steps or resolution forthis current loop communication is quantized by the division of the 16mA range between 4 and 20 mA. In early applications, loop currentresolution often did not exceed 100 steps, with 500 steps considered tobe high. More recent sensing and control applications can demandresolution of 1000 steps or more (requiring 16 microamps (uA) or lessper step). Effectively communicating information at these resolutionslevies increased requirements upon the capability of the transmitter toregulate and constrain current noise on the loop to peak-to-peaktolerances well below the resolution current, typically less than ½ theresolution current (as an example: ≤8 uA for 1000 step resolution).Additionally, digital communications protocols, such as HART™ whichsuperimposes frequency shift keying sinusoids at 1200 and 2200 Hz at anamplitude of 1 mA peak-to-peak AC current on the standard loop current,levy further demands on the bandwidth and stability capabilities of thetransmitter loop current control.

The functionality and internal power requirements for 2-wiretransmitters have evolved over the years. Early transmitters supportedsimple peripherals and circuits capable of operating within the loadpower restrictions imposed by the legacy transmitter power distributiontopology described above. Additionally, the continuous operation natureof these early transmitter circuits inherently limited the bandwidth ofnoise and transients drawn on I2 to at or below a cutoff frequency(fctrl) of the main loop control circuit such that these components didnot interfere with the transmitters ability to achieve the ILoopresolutions demanded at that time. Successive generations oftransmitters sought to utilize advances in low power components such asmicroprocessors, display technologies, circuit components and sensors toincrease functionality and features while retaining the power budgetsand frequency component constraints on I2 of earlier products permittingcontinued use of linear regulators to satisfy both power and loopcurrent noise requirements.

The recent proliferation of wireless communications in process andmonitoring environments and advances in low-power electronics andsensors have driven interest in incorporating these technologies toexpand the capabilities and features of 2-wire transmitter connecteddevices. Rather than operate in a continuously low power mode, many ofthese new technologies operate in burst or pulse modes in which thedevice alternates between longer intervals of low power demand andshorter intervals of high power demand to achieve a net low averagepower demand over an operational time period or cycle. In contrast tothe pulse loads demanded from such devices, the 2-wire transmitter isfundamentally limited in both peak magnitude and response time todeliver power to internal loads. Such limitations arise from the limitedpower availability presented from the external loop to the 4-20 mA port,the transmitter's internal circuit losses, response time limitations intransferring available port power to the load, and the need to constraintransients from the transmitter's internal power circuits from imposingnoise and disturbances on ILoop beyond the deviation limits required forcommunicating high resolution information on the loop. To successfullysupply components and systems with fast transient or pulse mode powerdemands requires the transmitter's internal power system be capable ofacquiring sufficient power available from the loop in a manner compliantwith the signal and resolution requirements for loop communication.Moreover, the system must further be capable of transferring,transforming and distributing this acquired power to meet the individualdemands of each active load and the combined load of all circuitssupplied by the transmitter.

Previously available systems have inadequately addressed therequirements and concerns outlined above. In one such system, a currentlimiter was used in parallel connection with a main loop controller tolimit the maximum current I2 that can be drawn by loads supplied by thetransmitter from the loop current. The set current limit was furtheradjustable by a connected microcontroller. While such an approach canlimit the maximum peak current that may be dynamically drawn bytransmitter loads, it does not limit transient load demands with peakcurrent below the set current limit. Load transients having magnitudesunder the current limit setpoint can have bandwidth and magnitude thatexceed the main loop controllers bandwidth and the ability thereof tosuppress the transients from being transferred as noise onto ILoop.

In another system, an adjustable linear voltage regulator was placed inseries with a switching power regulator which were together placed inparallel with the main loop controller. An indicated function of thelinear voltage regulator was to prevent noise generated by the switchingregulator from propagating to ILoop. However, use of a linear voltageregulator in this role cannot effectively control current or currentnoise drawn through the regulator (I2).

SUMMARY

In one aspect, a system includes a two-conductor loop in electricalconnection with a power source and with a loop current controller. Theloop current controller includes a first port having a first terminaland a second terminal to connect to respective conductors of thetwo-conductor loop and a second port in operative connection with asensor. The loop current controller controls a current in thetwo-conductor loop to be equal to a current signal to transmit thecurrent signal to a receiver connected to the two-conductor loop. Thecurrent in the two-conductor loop or current signal is controlled by theloop current controller to be proportional to a signal output from thesensor. The system further includes energy harvesting circuitry inelectrical connection with the two-conductor loop. The energy harvestingcircuitry includes a second current controller in parallel electricalconnection with the loop current controller and a power converter inelectrical connection with the second current controller. The secondcurrent controller controls a portion of current drawn from thetwo-conductor loop and delivered to the power converter from an outputport of the second current controller. The portion of the current drawnfrom the two-conductor loop is less than the current signal into aninput port thereof. The power converter converts the primary current anda primary voltage to a secondary current and a predetermined secondaryvoltage to be supplied to at least one load device. The portion of thecurrent drawn from the two-conductor loop is returned to the loopcurrent controller from the energy harvesting circuit. Noise in theportion of the current drawn from the two-conductor loop by the secondcurrent controller is controlled by the second current controller to bebelow a predetermined threshold so that the loop current controllercontrols noise in the current in the two-conductor loop to be less thana current corresponding to the resolution of the sensor. In a number ofembodiments, noise in the portion of the current drawn from thetwo-conductor loop to the secondary current controller is controlled sothat the loop current controller controls noise in the current in thetwo-conductor loop to be less than a current corresponding to one halfthe resolution of the sensor.

In a number of embodiments, the second current controller has asecondary bandwidth and the loop current controller has a firstbandwidth, and the secondary bandwidth is greater than the firstbandwidth. The secondary bandwidth may, for example, be sufficient toresist variation in the portion of current drawn from the two-conductorloop in response to variation in the primary current and the primaryvoltage arising in the energy harvesting circuit. The second currentcontroller may, for example, reject noise in the current drawn by theenergy harvesting circuit from the two-conductor loop such that thefirst bandwidth of the loop current controller is sufficient to restrictnoise in the current signal to be within a desired or predeterminedrange (for example, within one half of the resolution of the sensor). Ina number of embodiments, the second current controller rejects noise inthe current drawn by the energy harvesting circuit from thetwo-conductor loop such that the first bandwidth of the loop currentcontroller is sufficient to restrict noise in the loop current signal tobe 8 μA or less. The two-conductor loop may, for example, have ameasurement range in the current signal of 4-20 mA.

In a number of embodiments, the system further includes a processorsystem in operative connection with the sensor to receive an analogsignal from the sensor. The processor system converts the analog signalto a digital signal to be delivered to the loop current controller toestablish a first setpoint equivalent to the current signal. Theprocessor system may, for example, be in operative connection with thesecond current controller to set a second setpoint equal the portion ofthe current drawn from the two-conductor loop by the second currentcontroller. The second setpoint may, for example, be set based upon datafeedback to the processor system indicating a status of powerconversion.

In a number of embodiments, the loop current controller draws a currentI1 from the two-conductor loop such that I1 plus the returned portion ofthe current drawn from the two-conductor loop by the second currentcontroller is equal to the first setpoint, wherein the second setpointis always less than the first setpoint. The second setpoint may, forexample, be controlled by the processor system to be constant or to varyin a determined manner within the first bandwidth.

In a number of embodiments, the primary current is controlled tomaintain the power input to the power converter in a predeterminedmanner based upon data feedback to the processor system indicating astatus of power conversion.

The portion of current drawn from the two-conductor loop by the secondcurrent controller is further constrained by a current required tooperate the loop current controller and a current requirement of anycomponent other than the energy harvesting circuitry drawing currentfrom the two-conductor loop. The second setpoint further determines theamount of current allocated to the loop current controller to controlthe current signal via drawing current I1 from the two-conductor loop.

The portion of the current drawn from the two-conductor loop may, forexample, be determined by the processor system on the basis of the firstsetpoint and a voltage across the first port. The portion of the currentdrawn from the two-conductor loop may, for example, be determined by theprocessor system further on the basis of a power requirement of the atleast one load device and an efficiency of the power converter.

In a number of embodiments, the energy harvesting system furtherincludes a third current controller having an input port in electricalconnection with an output of the power converter. The third currentcontroller determines the secondary current drawn through the powerconverter. The energy harvesting system may further include at least oneenergy storage component which is in electrical connection with theoutput of the power converter and with the at least one load. The energystorage system may, for example, be in electrical connection with anoutput of the third current controller and in electrical connection withthe at least one load device.

The at least one energy storage component may, for example, include atleast one high-capacity capacitor. Energy may, for example, betransferred to the at least one load device from the at least onehigh-capacity capacitor if an instantaneous power requirement of the atleast one load device exceeds an instantaneous power available from thepower converter. Energy may, for example, be transferred to the at leastone high-capacity capacitor if the instantaneous power required by theat least one load device is less than the instantaneous power energyavailable from the power converter. In a number of embodiments, energymay be transferred to the at least one load device from the at least onehigh-capacity capacitor if the primary voltage is below a predeterminedthreshold.

In a number of embodiments, the processor system is in operativeconnection with the third current controller, determines a thirdsetpoint equivalent to the secondary current to be drawn through thepower converter and transmits the third setpoint to the third currentcontroller. In a number of embodiments, the processor system furthercontrols energy transfer from the at least one high-capacity capacitor.

The third setpoint may, for example, be determined by the processorsystem on the basis of at least one of a power available to the powerconverter and a predetermined operating range of the primary current andthe primary voltage. The second setpoint and the third setpoint may, forexample, be determined to maintain each of the primary current and theprimary voltage in a predetermined range. The second setpoint and thethird setpoint may, for example, be determined to control the operationof the power converter so that primary current noise generated by theconverter remains within the combined bandwidth of the second controllerand the converter input capacitance so that the primary current noisedoes not appear on I2.

In a number of embodiments, a capacity of the at least one high-capacitycapacitor is determined based upon a predicted load profile of the atleast one load device.

In a number of embodiments, the at least one load device is acombustible gas sensor. The combustible gas sensor may further be thesensor from which the signal is output.

The combustible gas sensor may, for example, include a first element,the first element including a first electric heating element, a firstsupport structure on the first electric heating element and a firstcatalyst supported on the first support structure. The combustible gassensor may further include electronic circuitry in electrical connectionwith the energy harvesting system and with the first element. Theelectronic circuitry provides energy to the first element to heat thefirst element to at least a first temperature at which the firstcatalyst catalyzed combustion of an analyte gas. The electroniccircuitry may, for example, apply energy to the first element in apulsed manner. The first element has a thermal time constant less than 8seconds, less than 6 seconds, less than 1 second, less than 0.5 secondsor less than 0.250 seconds.

The system may, for example, include a plurality of load devicesincluding a combustible gas sensor in operative connection with thepower conversion system. A capacity of the at least one high-capacitycapacitor may, for example, be determined based upon a predicted loadprofile of the plurality of load devices.

The processor system may, for example, control application of power toat least one of the plurality of load devices based upon a predeterminedrule set.

In a number of embodiments, the power converter includes a DC-DCswitched-mode power supply to provide a predetermined secondary voltage.In a number of embodiments, the power converter includes a buckconverter, a single-ended primary-inductor converter (SEPIC converter)or a switched capacitor converter to provide a predetermined secondaryvoltage.

In a number of embodiments, the primary current and the secondarycurrent are controlled to reduce or eliminate the need for largecapacitance required at an input of the power converter.

As described above, in a number of embodiments (whether the systemincludes the third current controller or not), the energy harvestingsystem further includes at least one energy storage component inelectrical connection with an output of the power converter and inelectrical connection with the at least one load device. The at leastone energy storage component may, for example, include at least onehigh-capacity capacitor. In a number of embodiments, energy istransferred to the at least one load device from the at least onehigh-capacity capacitor if an instantaneous power requirement of the atleast one load device exceeds an instantaneous power available from thepower converter.

In a number of embodiments, an impedance looking into the output of thesecond current controller is greater than an impedance looking into abypass capacitance in electrical connection of the output of the secondcurrent controller and with the input of the power converter. Theimpedance looking into the output of the second current controller may,for example, be sufficiently greater than the impedance of the inputlooking into the bypass capacitance (at frequencies at or above thebandwidth of the second controller) so that harmonics above thebandwidth of the second controller are drawn from the bypasscapacitance.

In another aspect, a method of harvesting energy from a two-conductorloop in electrical connection with a power source and with a loopcurrent controller is provided. The loop current controller includes afirst port having a first terminal and a second terminal to connect torespective conductors of the two-conductor loop and a second port inoperative connection with a sensor. The loop current controller controlsa current in the two-conductor loop to be equal to a current signal totransmit the current signal to a receiver connected to the two-conductorloop. The current in the two-conductor loop (current signal) iscontrolled by the loop current controller to be proportional to a signaloutput from the sensor. The method includes electrically connecting anenergy harvesting circuit to the two-conductor loop, wherein the energyharvesting circuit includes a second current controller connected inparallel electrical connection with the loop current controller and apower converter in electrical connection with an output of the secondcurrent controller. The second current controller controls a portion ofcurrent drawn from the two-conductor loop and delivered to the powerconverter from an output port of the second current controller. Theportion of the current drawn from the two-conductor loop is less thanthe current signal into an input port thereof. The method furtherincludes converting a primary current and a primary voltage to asecondary current and a predetermined secondary voltage to be suppliedto at least one load device via the power converter, and returning theportion of the current drawn from the two-conductor loop to the loopcurrent controller from the energy harvesting circuit. The noise in theportion of the current drawn from the two-conductor loop by the secondcurrent controller is controlled by the second current controller to bebelow a predetermined threshold so that the loop current controllercontrols noise in the current in the two-conductor loop to be less thana current corresponding to the resolution of the sensor.

As described above, in a number of embodiments, the second currentcontroller has a secondary bandwidth and the loop current controller hasa first bandwidth, and the secondary bandwidth is greater than the firstbandwidth. The secondary bandwidth may, for example, be sufficient toresist variation in the portion of current drawn from the two-conductorloop in response to variation in the primary current and the primaryvoltage arising in the energy harvesting circuit. The second currentcontroller may, for example, reject noise in the current drawn by theenergy harvesting circuit from the two-conductor loop such that thefirst bandwidth of the loop current controller is sufficient to restrictnoise in the current signal to be within a desired or predeterminedrange (for example, within one half of the resolution of the sensor). Ina number of embodiments, the second current controller rejects noise inthe current drawn by the energy harvesting circuit from thetwo-conductor loop such that the first bandwidth of the loop currentcontroller is sufficient to restrict noise in the loop current signal tobe 8 μA or less. The two-conductor loop may, for example, have ameasurement range in the current signal of 4-20 mA.

In a number of embodiments, the method further includes providing aprocessor system in operative connection with the sensor to receive ananalog signal from the sensor. The processor system converts the analogsignal to a digital signal to be delivered to the loop currentcontroller to establish a first setpoint equivalent to the currentsignal. The processor system may, for example, be in operativeconnection with the second current controller to set a second setpointequal to the portion of the current drawn from the two-conductor loop bythe second current controller. The second setpoint may, for example, beset based upon data feedback to the processor system indicating a statusof power conversion.

In a number of embodiments, the loop current controller draws a currentI1 from the two-conductor loop such that I1 plus the returned portion ofthe current drawn from the two-conductor loop by the second currentcontroller is equal to the first setpoint, wherein the second setpointis always less than the first setpoint. The second setpoint may, forexample, be controlled by the processor system to be constant or to varyin a determined manner within the first bandwidth.

In a number of embodiments, the primary current is controlled tomaintain the power input to the power converter in a predeterminedmanner based upon data feedback to the processor system indicating astatus of power conversion.

The portion of current drawn from the two-conductor loop by the secondcurrent controller is further constrained by a current required tooperate the loop current controller and a current requirement of anycomponent other than the energy harvesting circuitry drawing currentfrom the two-conductor loop. The second setpoint further determines theamount of current allocated to the loop current controller to controlthe current signal via drawing current I1 from the two-conductor loop.

The portion of the current drawn from the two-conductor loop may, forexample, be determined by the processor system on the basis of the firstsetpoint and a voltage across the first port. The portion of the currentdrawn from the two-conductor loop may, for example, be determined by theprocessor system further on the basis of a power requirement of the atleast one load device and an efficiency of the power converter.

In a number of embodiments, the energy harvesting system furtherincludes a third current controller having an input port in electricalconnection with an output of the power converter. The third currentcontroller determines the secondary current drawn through the powerconverter. The energy harvesting system may further include at least oneenergy storage component which is in electrical connection with the atleast one load. The energy storage system may, for example, be inelectrical connection with an output of the third current controller andin electrical connection with the at least one load device.

The at least one energy storage component may, for example, include atleast one high-capacity capacitor. Energy may, for example, betransferred to the at least one load device from the at least onehigh-capacity capacitor if an instantaneous power requirement of the atleast one load device exceeds an instantaneous power available from thepower converter. Energy may, for example, be transferred to the at leastone high-capacity capacitor if the instantaneous power required by theat least one load device is less than the instantaneous power energyavailable from the power converter. In a number of embodiments, energymay be transferred to the at least one load device from the at least onehigh-capacity capacitor if the primary voltage is below a predeterminedthreshold.

In a number of embodiments, the processor system is in operativeconnection with the third current controller, determines a thirdsetpoint equivalent to the secondary current to be drawn through thepower converter and transmits the third setpoint to the third currentcontroller. In a number of embodiments, the processor system furthercontrols energy transfer from the at least one high-capacity capacitor.

The third setpoint may, for example, be determined by the processorsystem on the basis of at least one of a power available to the powerconverter and a predetermined operating range of the primary current andthe primary voltage. The second setpoint and the third setpoint may, forexample, be determined to maintain each of the primary current and theprimary voltage in a predetermined range. The second setpoint and thethird setpoint may, for example, be determined to control the operationof the power converter so that primary current noise generated by theconverter remains within the combined bandwidth of the second controllerand the converter input capacitance so that the primary current noisedoes not appear on I2.

In a number of embodiments, a capacity of the at least one high-capacitycapacitor is determined based upon a predicted load profile of the atleast one load device.

In a number of embodiments, the at least one load device is acombustible gas sensor. The combustible gas sensor may further be thesensor from which the signal is output.

The combustible gas sensor may, for example, include a first element,which includes a first electric heating element, a first supportstructure on the first electric heating element and a first catalystsupported on the first support structure. The combustible gas sensor mayfurther include electronic circuitry in electrical connection with theenergy harvesting system and with the first element. The electroniccircuitry provides energy to the first element to heat the first elementto at least a first temperature at which the first catalyst catalyzedcombustion of an analyte gas. The electronic circuitry may, for example,apply energy to the first element in a pulsed manner. The first elementmay, for example, have a thermal time constant less than 8 seconds, lessthan 6 seconds, less than 1 second, less than 0.5 seconds or less than0.250 seconds.

The system may, for example, include a plurality of load devicesincluding a combustible gas sensor in operative connection with thepower conversion system. A capacity of the at least one high-capacitycapacitor may, for example, be determined based upon a predicted loadprofile of the plurality of load devices.

The processor system may, for example, control application of power toat least one of the plurality of load devices based upon a predeterminedrule set.

In a number of embodiments, the power converter includes a DC-DCswitched-mode power supply to provide a predetermined secondary voltage.In a number of embodiments, the power converter includes a buckconverter, a SEPIC converter or a switched capacitor converter toprovide a predetermined secondary voltage.

In a number of embodiments, the primary current and the secondarycurrent are controlled to reduce or eliminate the need for largecapacitance required at an input of the power converter.

As described above, in a number of embodiments (whether a third currentcontroller is provided or not), the energy harvesting system may furtherinclude at least one energy storage component in electrical connectionwith an output of the power converter and in electrical connection withthe at least one load device. The at least one energy storage componentmay, for example, include at least one high-capacity capacitor. In anumber of embodiments, energy is transferred to the at least one loaddevice from the at least one high-capacity capacitor if an instantaneouspower requirement of the at least one load device exceeds aninstantaneous power available from the power converter.

As also described above, in a number of embodiments, an impedancelooking into the output of the second current controller is greater thanan impedance looking into a bypass capacitance in electrical connectionof the output of the second current controller and with the input of thepower converter. The impedance looking into the output of the secondcurrent controller may, for example, be sufficiently greater than theimpedance of the input looking into the bypass capacitance (atfrequencies at or above the bandwidth of the second controller) so thatharmonics above the bandwidth of the second controller are drawn fromthe bypass capacitance.

In a further aspect, a system includes a two-conductor loop inelectrical connection with a power source and with a loop currentcontroller. The loop current controller includes a first port having afirst terminal and a second terminal to connect to respective conductorsof the two-conductor loop and a second port in operative connection witha combustible gas sensor. The loop current controller controls a currentin the two-conductor loop to be equal to a current signal to transmitthe current signal to a receiver connected to the two-conductor loop.The current in the two-conductor current loop is controlled by the loopcurrent controller to be proportional to a signal output from thecombustible gas sensor. The system further includes an energy harvestingcircuitry in electrical connection with the two-conductor loop. Theenergy harvesting circuitry includes a second current controller inparallel electrical connection with the loop current controller and apower converter in electrical connection with the second currentcontroller. The second current controller controls a portion of currentdrawn from the two-conductor loop and delivered to the power converterfrom an output port of the second current controller. The portion of thecurrent drawn from the two-conductor loop is less than the currentsignal into an input port thereof. The power converter converts aprimary current and a primary voltage to a secondary current and apredetermined secondary voltage to be supplied to at least one loaddevice. The portion of the current drawn from the two-conductor loop isreturned to the loop current controller from the energy harvestingcircuit. The noise in the portion of the current drawn from thetwo-conductor loop by the second current controller is controlled by thesecond current controller to be below a predetermined threshold so thatthe loop current controller controls noise in the current in thetwo-conductor loop to be less than a current corresponding to theresolution of the combustible gas sensor. The system may, for example,be otherwise characterized as described above.

The present devices, systems, and methods, along with the attributes andattendant advantages thereof, will best be appreciated and understood inview of the following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates embodiment of currently available 2-wire 3-wire and4-wire current loops.

FIG. 1B illustrates a representative embodiment of a currently available2-wire current loop system.

FIG. 1C illustrates operation and limitations of the current controlcircuit or current controller of a transmitter in restricting sources ofvariability or noise in the current on the loop.

FIG. 1D illustrates an overlay of power converter (for example, aswitched mode power supply or SMPS) input current harmonics on the first(or main loop) current controller rejection capabilities.

FIG. 2A illustrates a high-level block diagram of a representativeembodiment of a 2-wire current loop hereof including an energyharvesting system in which sources of variability or noise in thecurrent on the loop are restricted.

FIG. 2B illustrates a more detailed block diagram of a representativeembodiment of a 2-wire current loop hereof including a second currentcontroller in parallel electrical connection with a first currentcontroller (that is, the main loop current controller of a current looptransmitter) and with a power converter in which sources of variabilityor noise in the current on the loop are restricted.

FIG. 2C illustrates a more detailed block diagram of a representativeembodiment of a 2-wire current loop hereof including an energyharvesting system further including an energy storage system and a thirdcurrent controller to control Isecondary in which sources of variabilityor noise in the current on the loop are restricted.

FIG. 2D illustrates another block diagram of a representative embodimentof a 2-wire current loop hereof including an energy harvesting systemfurther including an energy storage system.

FIG. 2E illustrates another block diagram of a representative embodimentof a 2-wire current loop hereof partitioned for analysis of loads on thesystem and power requirements.

FIG. 3A sets forth as subset of Maxwell's equations and the derivationof Kirchoff's law from Ampere's law.

FIG. 3B illustrates an embodiment of a second current controller and apower converter hereof, identifying currents and impedances referencedin FIG. 3C, and identifies impacts of the second and third currentcontrollers on capacitor size (capacity) and transient charging of thepower converter primary capacitors.

FIG. 3C illustrates the role of the high bandwidth second currentcontroller with high output impedance in forcing the harmonic currentdrawn by the power converter primary to be drawn from the primary bypasscapacitors, thereby attenuating and rejecting such harmonics from beingreturned to the low bandwidth first controller (loop controller).

FIG. 4 illustrates an embodiment of a system here of where the setpointcontrol of the second current controller uses an analog filter torestrict the rate of change of the setpoint to within the bandwidthcapabilities of the first controller.

FIG. 5 illustrates load profiles which are combined into a compositesecondary load profile to determine an overall energy use profile.

FIG. 6A (in combination with FIGS. 6B through 6E) illustrates theoperation of the first and second current controllers to manage thepower converter primary voltage during ILoop transients and the combinedaction of the third current controller and load switching to increaseand decrease the demand on Iprimary (through Isecondary) to both captureexcess energy and return it to the loads to make optimal use ofavailable power at the power converter primary and sustain secondarypower needs when Pprimary falls below PprimaryReq, wherein FIG. 6Aillustrates such functionality of with varying power from the port.

FIG. 6B illustrates the functionality described in connection with FIG.6A wherein variation in stored charge is shown.

FIG. 6C illustrates the functionality of the third current controllerand load switching to increase and decrease the demand on Iprimary(through Isecondary) to both capture excess energy and return it to theloads to make optimal use of available power.

FIG. 6D illustrate the functionality described in connection with FIG.6A wherein variation in current is shown.

FIG. 6E illustrates a block diagram of the energy harvester inidentifying features and variables associated with FIGS. 6A through 6D.

FIG. 7 illustrates an embodiment of a system herein wherein the outputof the third current controller is connected to a charge pump thatcharges storage capacitors.

FIG. 8 illustrates an embodiment of a system hereof wherein the outputof the third current controller is connected directly to the storagecapacitor(s).

FIG. 9 illustrates an embodiment of a system hereof in which energystorage is be provided on the primary side of the power converter (suchthat the system may direct a portion of the current from the secondarycurrent controller to one or more additional capacitors in parallel withthe one or more capacitors at the primary side of the power converter)wherein, while Vprimary is discharging to below Vlimited and I2 isshunted to Ilimit and stored in capacitors Cpstore1 and 2.

FIG. 10 illustrates an embodiment of a system wherein current istransferred via a boost converter from Ccapture to Cpstore1 and 2.

FIG. 11A (in combination with FIGS. 11B and 11C) illustrates a model forestimation of parameters available and required power for an embodimentof model-based control, wherein FIG. 11A illustrates an embodiment ofthe circuit.

FIG. 11B illustrates an ideal steady state port, primary and powerconverter circuit model.

FIG. 11C illustrates an ideal steady port power estimation model.

FIG. 12 illustrates equations for estimation and control of powerparameters.

FIG. 13 illustrates an embodiment of a system hereof, sometime referredto as a “push system” wherein the second current controller “pushes”available power/energy from P1 to Pprim.

FIG. 14 illustrates an embodiment of a system hereof, sometime referredto as a “pull system” wherein the third current controller “pulls”available power/energy from the primary Pprim to the secondary Psec anddrives excess secondary current Isec to storage.

FIG. 15 illustrates an embodiment of a system hereof in which the secondand third current controllers are operated as described in FIGS. 13 and14 in a combined/synchronized operation which may, for example, bereferred to as a “push-pull” methodology.

FIG. 16A illustrates an embodiment of a port power estimation model useto project transmitter load and power converter requirements to thetransmitter port.

FIG. 16B illustrates a steady-state external loop model to assess andestimate power available to the transmitter port.

FIG. 16C illustrates available port voltage VportAv vs ILoop for threedifferent loops, wherein interrogation setpoints are marked by “X”.

FIG. 16D illustrates load projections to port voltage (Vport_(Req)) vsILoop requirements for transmitter using three different internal powerconverters.

FIG. 16E illustrates an operational capability assessment of atransmitter with Vport_(Req) connected alternatively to a loop with 500ohm and 800 ohm load resistances, wherein the “X” represents theintersection of the 800 ohm load line with Vport_(Req), indicating thatthe loop is incapable of supporting the power required by thetransmitter.

FIG. 17A illustrates primary and port voltage characteristics vs ILoopfor a first Buck converter.

FIG. 17B illustrates primary and port current characteristics vs ILoopfor the first Buck converter.

FIG. 17C illustrates primary and transmitter port power characteristicsfor the first Buck converter.

FIG. 17D illustrates SMPS efficiency (η) required to produce uniformpower equal to power at ILoopMax across the entire operating range ofILoop.

FIG. 18 illustrates schematically a circuit representation of a Buckconverter.

FIG. 19 illustrates schematically a circuit representation of a SEPICconverter.

FIG. 20A illustrates SEPIC SMPS primary and port voltage characteristicsvs ILoop.

FIG. 20B illustrates a comparison of Vport_(Req) for three SMPS designs(a first Buck converter, a second Buck converter, and a SEPIC converter)at 3.5 and 22 mA.

FIG. 21A illustrates a worst case load estimate for a transmitterincluding an electrochemical gas sensor connected thereto.

FIG. 21B illustrates the electrochemical gas sensor dynamic current at a1 second sampling interval for the electrochemical gas sensor include inthe estimate of FIG. 21A.

FIG. 22A illustrates an embodiment of a representative load in the formof a reed relay and magnet user input interface and an estimate ofassociated required secondary current.

FIG. 22B illustrates port voltage requirements vs. loop current (Iport)with the magnet user input interface of FIG. 22A.

FIG. 22C illustrates port load capability vs loop supply voltage withthe magnet input interface of FIG. 22A.

FIG. 22D illustrates loop length capability with a 24V supply and a 250ohm measurement resistor with the magnet input interface of FIG. 22A.

FIG. 22E illustrates Table 2 which sets forth tabulated requirements andcapability of a transmitter including the magnet input interface of FIG.22A.

FIG. 23A illustrates an embodiment of a conventional combustible gassensor.

FIG. 23B illustrates an enlarged view of a Wheatstone bridge circuitincorporating the sensing element and the compensating element of thecombustible gas sensor of FIG. 23A.

FIG. 24A illustrates an embodiment of a MEMS pellistor for use in thesystems hereof.

FIG. 24B illustrates a MEMS sensor mounted on a printed circuit board.

FIG. 25 illustrates a response profile at various concentration ofmethane for a commercially available microhotplate or MEMS device.

FIG. 26 illustrates Table 4 which sets forth methane operatingconditions and response data for a microhotplate or MEMS device.

FIG. 27 illustrates Table 5 which sets forth methane operatingconditions and response data for another sensor.

DETAILED DESCRIPTION

It will be readily understood that the components of the embodiments, asgenerally described and illustrated in the figures herein, may bearranged and designed in a wide variety of different configurations inaddition to the described representative embodiments. Thus, thefollowing more detailed description of the representative embodiments,as illustrated in the figures, is not intended to limit the scope of theembodiments, as claimed, but is merely illustrative of representativeembodiments.

Reference throughout this specification to “one embodiment” or “anembodiment” (or the like) means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearance of the phrases “in oneembodiment” or “in an embodiment” or the like in various placesthroughout this specification are not necessarily all referring to thesame embodiment.

Furthermore, described features, structures, or characteristics may becombined in any suitable manner in one or more embodiments. In thefollowing description, numerous specific details are provided to give athorough understanding of embodiments. One skilled in the relevant artwill recognize, however, that the various embodiments can be practicedwithout one or more of the specific details, or with other methods,components, materials, et cetera. In other instances, well knownstructures, materials, or operations are not shown or described indetail to avoid obfuscation.

As used herein and in the appended claims, the singular forms “a,” “an”,and “the” include plural references unless the context clearly dictatesotherwise. Thus, for example, reference to “a load” includes a pluralityof such loads and equivalents thereof known to those skilled in the art,and so forth, and reference to “the load” is a reference to one or moresuch loads and equivalents thereof known to those skilled in the art,and so forth. Recitation of ranges of values herein are merely intendedto serve as a shorthand method of referring individually to eachseparate value falling within the range. Unless otherwise indicatedherein, and each separate value, as well as intermediate ranges, areincorporated into the specification as if individually recited herein.All methods described herein can be performed in any suitable orderunless otherwise indicated herein or otherwise clearly contraindicatedby the text.

The terms “electronic circuitry”, “circuitry” or “circuit,” as usedherein include, but is not limited to, hardware, firmware, software orcombinations of each to perform a function(s) or an action(s). Forexample, based on a desired feature or need. a circuit may include asoftware controlled microprocessor, discrete logic such as anapplication specific integrated circuit (ASIC), or other programmedlogic device. A circuit may also be fully embodied as software. As usedherein, “circuit” is considered synonymous with “logic.” The term“logic”, as used herein includes, but is not limited to, hardware,firmware, software or combinations of each to perform a function(s) oran action(s), or to cause a function or action from another component.For example, based on a desired application or need, logic may include asoftware controlled microprocessor, discrete logic such as anapplication specific integrated circuit (ASIC), or other programmedlogic device. Logic may also be fully embodied as software.

The term “processor,” as used herein includes, but is not limited to,one or more of virtually any number of processor systems or stand-aloneprocessors, such as microprocessors, microcontrollers, centralprocessing units (CPUs), and digital signal processors (DSPs), in anycombination. The processor may be associated with various other circuitsthat support operation of the processor, such as random access memory(RAM), read-only memory (ROM), programmable read-only memory (PROM),erasable programmable read only memory (EPROM), clocks, decoders, memorycontrollers, or interrupt controllers, etc. These support circuits maybe internal or external to the processor or its associated electronicpackaging. The support circuits are in operative communication with theprocessor. The support circuits are not necessarily shown separate fromthe processor in block diagrams or other drawings.

The term “controller,” as used herein includes, but is not limited to,any circuit or device that coordinates and controls the operation of oneor more input and/or output devices. A controller may, for example,include a device having one or more processors, microprocessors, orcentral processing units capable of being programmed to performfunctions.

The term “logic,” as used herein includes, but is not limited to.hardware, firmware, software or combinations thereof to perform afunction(s) or an action(s), or to cause a function or action fromanother element or component. Based on a certain application or need,logic may, for example, include a software controlled microprocess,discrete logic such as an application specific integrated circuit(ASIC), or other programmed logic device. Logic may also be fullyembodied as software. As used herein, the term “logic” is consideredsynonymous with the term “circuit.”

The term “software,” as used herein includes, but is not limited to, oneor more computer readable or executable instructions that cause acomputer or other electronic device to perform functions, actions, orbehave in a desired manner. The instructions may be embodied in variousforms such as routines, algorithms, modules or programs includingseparate applications or code from dynamically linked libraries.Software may also be implemented in various forms such as a stand-aloneprogram, a function call, a servlet, an applet, instructions stored in amemory, part of an operating system or other type of executableinstructions. It will be appreciated by one of ordinary skill in the artthat the form of software is dependent on, for example, requirements ofa desired application, the environment it runs on, or the desires of adesigner/programmer or the like.

The term “power converter” as used herein refers to system such as anelectronic circuit or electromechanical device that converts a source ofdirect current or DC from one voltage level to another. The term“current controller” as used herein refers to an electronic circuit orelectromechanical device that draws from a source a predeterminedcurrent in response to a supplied setpoint.

In a number of embodiments, devices, systems and methods hereof enable a2-wire transmitter to supply, for example, transients or intermittenthigh pulse power loads via a power converter from power acquired fromthe external loop while retaining the capability to communicate highresolution information on the loop. Use of a power converter in thesystems hereof provides a number of advantages as compared to the use ofa linear power regulator in previous systems. In the use of a linearpower regulator, the sustainable power available across the operatingrange of loop current is limited and defined by the product of theminimum loop current (which is coincident with the highest port voltage)and the minimum regulator input voltage which must be strictly greaterthan the output voltage and strictly less than the minimum voltageavailable from the loop (which is coincident with the lowest loopcurrent). The power available from input voltage in excess of theminimum is dissipated as heat in the regulator. However, use of powerconversion in the devices, systems and methods hereof, the input poweris defined by the product of the available input current and inputvoltage (above the minimum voltage required by the converter) to theconverter. That input power, which is lost as heat in the linearregulator, may be converted to additional sustainable secondary power tosupply loads across the operational range of loop current in the case ofa power converter. Although power converters are inherently noisy inoperation, the use of a second current controller in the devices,systems and methods hereof enables use of a power converter whileretaining the capability to communicate high resolution information onthe loop.

FIG. 1C illustrates the role and limitation of the transmitter's mainloop current control circuit in restricting sources of variability onILoop, most notably from the transmitter's internal power supply andcircuits powered from I2, to achieve the required resolution andcompliance of ILoop to ILoop_(setpoint). The aspects of the noise andtransients associated with providing adequate power for transmittercircuits from I2 while achieving compliance of ILoop to ILoop_(setpoint)within the variability defined by the required resolution share acodependence. This codependence is best understood by considering thecapabilities and limitations of the transmitter's loop currentcontroller (referenced in FIG. 1C as the main loop controller). FIG. 1Cillustrates three regions associated with the main loop controllerbandwidth. Within the pass band region bounded by DC (0 Hz) and cutofffrequency fctrl, the transmitter or main loop controller is capable ofmodulating I1 to hold ILoop=ILoop_(setpoint) while adequately rejectingnoise and transients drawn by the internal transmitter components andpower supply on I2. In practice, fctrl must be kept sufficiently low toavoid instability over the broadest range of impedance presented by theexternal components on the loop, yet high enough to support the fastestrequired loop transients such as those demanded by digitalcommunications protocols such as HART™. In the transition band abovefctrl, the loop controller becomes less capable of holding ILoop toILoop_(setpoint) and begins to allow components of I2 over thefrequencies in this band to be passed to ILoop until, at and above thestop band frequency fstop, the controller is no longer capable ofdriving I1 to hold ILoop compliant to ILoop_(setpoint), and componentsof I2 pass fully to ILoop. FIG. 1D illustrates an overlay of powerconverter (for example, a switched mode power supply or SMPS) inputcurrent harmonics on the first (or main loop) current controllerrejection capabilities.

As described above, a first current controller (sometimes referred toherein and/or in the figures as a main loop controller, loop currentcontroller or current controller 1) in a transmitter is used to controlthe current in a current loop of a two-wire system. As also describedabove, the first or loop current controller must have a narrow bandwidthto ensure stability in controlling the loop current while accommodatinga broad range of reactance/impedance presented by external components inelectrical connection with the loop (the, receiver etc.). Broadening thebandwidth of the first current controller sufficiently to counteractfast current transients drawn by pulse loads or a power converter wouldextend beyond the maximum bandwidth to reliably avoid oscillation onILoop as a result of the reactance/impedance of the external loopcomponents.

In the present devices systems and methods, a second current source orcurrent controller (sometimes referred to herein and/or in the figuresas current controller 2) is used to provide current to a power convertersuch as a switch mode power supply. To leverage the capabilities of thefirst current controller to achieve the resolution required of thecurrent loop requires that the amplitude of components of I2 berestricted in accordance with the frequency dependent I2 rejectioncapabilities of the first current controller. In a number of embodimentshereof, the second current controller is a high bandwidth currentcontroller, having a bandwidth greater than the (and typicallysignificantly greater than) the first current controller.

The second current controller, which, as illustrated in FIGS. 2A through2C, is in parallel electrical connection with the first currentcontroller and effectively takes control of the loop current. The secondcurrent controller provides or allocates the remainder of the loopcurrent to other circuit elements. The second current controller cannotbe in series with the first current controller as the currenttherethrough would be same. Further, it is not possible to varybandwidth with devices in series because the combination thereof willhave the bandwidth of the lower bandwidth device (that is, the bandwidthof the first current controller in this case).

The second current controller exerts control in driving I2 such that thenoise in the current I2 returned to the first current controller iswithin the bandwidth of the first current controller. The first currentcontroller can thus adequately control I1 in a manner to satisfy theresolution constraints of the current loop. In a number ofrepresentative embodiments discussed herein, the second current (ormain/loop) controller is discussed as a circuit component separate fromthe power converter. However, one skilled in the art will appreciatethat the second current controller may be integrated into the powerconverter.

In operation of the devices, systems and methods hereof, the secondcurrent controller draws a determined portion of the signal or loopcurrent. Because the second current controller has high bandwidth, itcan fundamentally control the portion of the current transmitted topower converter and allocate the remainder to the first currentcontroller and losses in current loop. The second controller does notfunction as a current limiter to restrict varying currents drawn by anycircuit or load connected to its output to a maximum limit. To thecontrary, the second controller actively drives I2 current in accordancewith the setpoint sent by the processor system/controller to arbitratethe portion of ILoop pushed to the energy harvesting circuitry andallocate the portion of ILoop reserved for the first controller,providing sufficient margin for the first controller to control ILoop.

Unlike the first current controller, which must employ limited bandwidthto guarantee stability, the second current controller need only closeupon its own fixed internal feedback. The second current controller canthus have a very wide bandwidth, which provides very stringent andprecise control over the current drawn from the loop (I2). Throughsubstitution and manipulation of Maxewell's fourth equation (also knownas Ampere's law—see FIG. 3A), one may derive an expression of Kirchoff'scurrent law that mandates that the sum of all currents entering andleaving a closed surface are equal to the time rate of chargeaccumulated inside that surface. In the operation of devices, systemsand methods hereof, the time rate of accumulated charge (−dQ/dt) ofcircuits supplied by the second current controller is negligible. Thus,as expressed by Kirchoff's current law, the sum of the current I2 andthe current returned to local circuit ground (GND_local in theillustrated embodiments) by all circuits supplied by the second currentcontroller must be zero. Therefore, the current returned on the localcircuit ground to the main loop controller from circuits supplied by thesecond current controller must equal I2. This concept is illustratedschematically in FIG. 3B by the I2 current entering the closed surfacedrawn around all circuits (indicated by the dashed line) supplied by thesecond current controller (including the power converter and allcircuits supplied by the power converter) is the same I2 current that isreturned on GND_local. Stated simply, the current I2, which is drawnthrough second current controller from the current loop, is returned tothe first current controller via the local ground. In other words, thesum of all currents returned to the first current controller fromcircuits supplied by the second current controller must sum to I2 underKirchoff's current law and more generally under Maxwell's law. Becauseof the high bandwidth and control capability of the second currentcontroller, the I2 current supplied to the power converter primary isvery low noise. By the relationship imposed by Kirchoff's current law,the current returned to the first current controller is thus also verylow noise. In a number of embodiments, the high bandwidth, secondcurrent controller is capable of modulating a pass transistor (MFet_I2in FIG. 3B) over the bandwidth of the second current controller todeliver a low noise I2 to a power converter despite the harmonics drawnby a power converter. The second current controller thereby prevents thenoise that would otherwise be associated with power conversion frombeing returned to the first current controller/current loop. Once again,any noise returned is at a level suitable to retain the resolutionrequired of the two-wire current loop. The bandwidth of second currentcontroller extends to frequencies significantly higher than the firstcurrent controller, that is, to frequencies wherein the noise from thepower converter occurs. The second current controller modulates itscircuitry to maintain low noise current I2 at the commanded setpoint sothat the current returned to the first current controller from the powerconverter and other elements beyond the second current controller isalso low noise and does not contain the noise from the power converterand other elements beyond the second current controller. As describedabove, the first control controller ceases to be able to control theloop current when the return current includes noise beyond its limitedbandwidth.

In a number of embodiments, the noise in ILoop, which is a combinationof noise in I2 and I1, is less than the resolution to retain thecapability to communicate high resolution information on the loop. In anumber of embodiments, the devices, systems and methods hereof restrictnoise (or maintain noise sufficiently low) in I2 such that noise inILoop is less than a predetermined threshold (for example, theresolution of the sensor or ½ the resolution of the sensor).

The current I2 drawn from the current loop by the second controller may,for example, be set to maintain the input requirements of the powerconverter input current and voltage, Iprimary and Vprimary, respectively(wherein, Vprimary is controlled via control of Iprimary). A setpoint(for example, 2 milliamp DC) for I2 to be drawn from the current loop bythe second current controller can be maintained regardless ofdisturbances that extend to a high frequency. The high bandwidth of thesecond current controller may be combined with a high output impedancethereof in maintaining low noise in current I2 regardless of the noisein current being drawn therefrom by the power converter. Inrepresentative embodiments, the first current controller may, forexample, have a first bandwidth of ten kilohertz, while the secondcurrent controller may, for example, have a second bandwidth of tenmegahertz or higher. The second current controller forces input currentharmonics drawn by the power converter that reside in the frequency band(bounded on the lower end by the maximum frequency generated by the stepsize and rate of setpoint changes applied to the controller and at theupper end by the maximum bandwidth of the controller) to be drawn fromthe input bypass capacitance of the power converter. For harmonics abovethe maximum bandwidth of the second current controller, the input bypassfor the power converter may be designed so that the impedance of thebypass appears sufficiently low compared to the impedance looking intothe output of the second current controller such that harmonics in thisband are drawn from the input bypass and are suppressed sufficientlythrough the second current controller (such that the residual currentnoise passed back to the first current controller falls is sufficientlylow to meet required resolution of the loop current). The alignment ofsecond controller output impedance and input bypass capacitance withpower converter input current harmonics for a representative embodimenthereof are illustrated in FIG. 3B. The effect of the high bandwidth andhigh impedance output of the second controller and the complimentaryrole of the low impedance bypass in the isolation and circulation ofharmonic current drawn from and returned to the power converter inputbypass capacitance (Cprimary*dVPrimary/dt) is illustrated in FIG. 3C. Ina number of embodiments, the impedance looking into the output of secondcurrent controller is significantly greater than the impedance of theinput bypass capacitance (for example, 5, 10, 100, or 1000 timesgreater). As illustrated in FIG. 3C, the impedance looking into theoutput of second current controller may, for example, be greater than 1kΩ and range to 1 GΩ over the frequency range of operation.

As an added benefit, the current noise suppression bandwidth of thesecond current controller relaxes the lower frequency impedancerequirement that would otherwise be imposed on the composite passivebypass components connected to the input of the power converter. Thisprovides opportunity to eliminate or reduce the size of these bypasscomponents, which tend to be physically larger at lower frequencies thanat the higher frequencies beyond the bandwidth of the second currentcontroller. The capability afforded by the second current controller toreduce capacitance at the input to the power converter enablessignificant improvement in maintaining optimal operating voltage at thepower converter primary as the control setpoint for I2 is varied toremain at the minimum required margin below Iloop as Iloop is varied.The capability to reduce the power converter input capacitance is mostsignificant when Iloop changes from a higher current with lower voltageacross the 2-wire port terminals to a lower current with higher voltageacross the 2-wire port terminals. Prior to the change in Iloop, thevoltage Vprimary at the power converter input must be at a lower value(less than Vport at the 2-wire terminals). As Iloop is decreased andVport rises, the Vprimary voltage at the input to the power convertermust be charged (increased) to transfer available power to the input ofthe converter. However, since I2 is constrained to be less than Iloop,the I2 current charging the power converter input capacitance islimited. Referring to FIG. 3B, the time (deltaT) required to raise thevoltage by an incremental amount (deltaVprimary) at the input to thepower converter is proportional to deltaT (deltaTdeltaVprimary*Cprimary/(I2−Iprimary). Cprimary is the total combinedparallel capacitance at the input to the power converter and Iprimary isthe average current drawn into the power converter over interval deltaT.Thus, the opportunity afforded by the second current controller toreduce Cprimary provides significant benefit in improving the Cprimarycharging time (deltaT) and enabling faster recovery and utilization ofavailable power by the power converter when Iloop modulates from high tolow current.

In a number of embodiments, the control system hereof (for example,including one or more processors and one or more associated memorysystems in communicative connection therewith) sets a setpoint for thecurrent to be drawn from the two-wire current loop by the second currentcontroller. The setpoint for the second current controller may, forexample, be fixed or slowly varying. Because the second controller hasmuch higher bandwidth than the first controller, the second controlleris capable of making changes to I2 (in response to changes in secondcontroller setpoint) faster than the first controller can react to andcompensate to maintain the ILoop setpoint. In a number of embodiments,the processor system limits the rate and magnitude of adjustments to thesecond controller setpoint so that the rate of change in secondcontroller setpoint falls within the rate of response capability of thefirst controller to adjust I1 to maintain ILoop within the noise limitgoal for ILoop. The setpoint of the second controller may, for example,be controlled in concert with changes made to the first controllersetpoint when modifying ILoop such that the rate and magnitude ofchanges in I2 in response to changes in the second controller setpointresult in a targeted allocation tolerance for the range and rate of I1drawn by the first controller to reach and maintain the ILoop setpoint.In a number of embodiments, the rate and magnitude of changes to thesetpoint for the second current controller is limited so that the rateand magnitude of corresponding changes in I2 remain within the bandwidthcapabilities of the first controller. In a number of embodiments, thesetpoint control of the second current controller uses an analog filterto restrict the rate of change of the setpoint to within the bandwidthcapabilities of the first controller as illustrated in FIG. 4.

In general, it is necessary to maintain a minimum allocation of ILoopcurrent to I1 to allow for things like digital communications modulationand variances in leakage currents prior to the second controller thatare returned to the local ground. In a number of embodiments, whenmaintaining a minimum allocation, the sequencing of ILoop setpoints andthe second controller setpoints may be important. For example, when theILoop setpoint decreases, the setpoint of the second controller may bereduced prior to reducing the ILoop setpoint. For rising changes inILoop, the ILoop setpoint change may precede any increase in the secondcurrent controller setpoint.

The bandwidth and output impedance requirements of the second currentcontroller relate back to or are determined by the bandwidth of inputtransients generated by the power converter device. In that regard,there is a fundamental frequency of the power converter. Moreover,harmonics arise based on switching frequency and primary current pulseshape and duration. The bandwidth of these harmonics is based on risetime, pulse width and pulse shape. Once again, the bandwidth and outputimpedance of the second current controller are suitable to resist thenoise in the current drawn by the power converter. As described furtherbelow, the system hereof may provide control the operational regions ofthe power converter to maintain the noise created thereby withinpredetermined or known bounds. In that regard, the systems hereof maycontrol the operation of the power converter (for example, via controlof the input or primary current and voltage as well as the output orsecondary current) so that noise created by the power converter stayswithin bounds.

The first current controller regulates the current in the loop (to beequal to the signal current) by drawings or pulling current I1. Thecontrol system establishes an internal set point for the signal currentbased on the sensor signal for the first current controller. The controlsystem also establishes a second set point (I2sp) for the second currentcontroller. The control system determines the current setpoint of thesecond controller based upon the current loop set point in addition toeither budgeted or measured current losses occurring between the 4-20 mAterminals and the second current controller (Iloss). The second setpoint establishes the current I2 drawn from the current loop andreturned to the first current controller. The second current controllerallocates what is left of the loop set point (that is, the 4-20 mAsetpoint) to the first current controller and Iloss. The control systemsets the second set point as a determined amount of the first or loopcurrent set point. The difference between the first set point and thesecond set point and Iloss is the amount of current that is allocated tothe first current controller to control the loop or signal current. Thecurrent I1 must be greater than 0 and, in a number of representativeembodiments, greater than approximately 0.5 milliamp because of overhead(for example, a Hart modem). To meet this constraint, the secondcontroller setpoint for I2 must not exceed the limit(I2setpoint<=Iloopsetpoint−I1min−Ilossmax, where I1min is the minimumcurrent allocated to I1 and Ilossmax is the maximum measured orallocated current for Iloss.

The second current controller thus may be considered to be a primary orprincipal current control component. The second current controller pullsor draws in a set amount of available current. Because of the higherbandwidth of the second current controller and the manner in which thesystem is configured, the second current controller controls the amountof current that can be used in the first current controller. The firstcurrent controller sets I1, but the second current controller sets 12.

In a number of embodiments, the devices, systems and methods hereofenable operation of high bandwidth and high pulse mode loads inconnection with a current loop, while restricting or limiting noise onthe current loop to achieve a desired resolution. High transient powerdemands and high resolution current control are achieved at the sametime.

In a number of embodiments of systems hereof, excess energy (that is,energy in excess of that required by the one or more loads to be poweredfrom energy harvested form the current loop) from the power converter isstored in an energy storage system which may, for example, include oneor more high capacity capacitors (sometimes referred to assupercapacitors, supercaps, or ultracapacitors). As known in theelectrical arts, supercapacitors may, for example, use electrostaticdouble-layer capacitance and electrochemical pseudocapacitance. Energystorage is, for example, very useful in cases when the energy/powerrequirements of the load(s) in electrical connection with the secondaryside of the power converter exceed the energy output of the powerconverter. For example, the instantaneous power requirements of apulse-operated and/or other device may exceed the instantaneous poweravailable from the power converter. Energy harvesting and storageprovides a strategy for dealing with circumstances in which theinstantaneous power required by the load(s) exceeds what is availablefrom the power converter. In cases wherein secondary energy exceeds thatrequired by the load, energy may be transferred to the energy storagesystem.

Given, for example, parameters such as predicted range of poweravailable from the current loop port, the efficiency of the powerconverter and predicted load requirements, one may determine energystorage requirements for a given load schedule. In general, one maydesign all circuits/loads to use as little power as possible. Each loaddevice has a predeterminable energy usage profile. Cycles and sequencesof loads for a composite energy usage profile may be determined. Asillustrated in FIG. 5 (in which load profiles for a microprocesser andcontrol electronics, a BLUETOOTH lower energy communication device and acombustible gas sensor are combined into a composite secondary loadprofile), one may, for example, compile such cycles and sequences ofloads to determine an overall energy use profile. Different profiles maybe constructed to enact distinct functions or modes of operation. Forexample, separate load sequencing profiles may be constructed forpower-up, to enact user commands or separate profiles may be constructedto perform the same group of tasks with tradeoffs to improve or reduceperformance at higher or lower required secondary current. Through loadscheduling, one may, for example, attempt to make required power remainconstant or vary slowly, which reduces the noise that would otherwise betransferred to the power converter primary via sudden changes inconverter operation in response to fast transient loads at the convertersecondary. Thus, in a number of embodiments of systems hereof, the loadsystem is determined at the time of manufacture of the system. Variousmodes of operation with various elements/loads drawing power may bepredetermined. Power is defined as the rate of producing or consumingenergy. Thus, power is energy per unit of time. Power requirements maybe determined in time intervals defined by such modes of operation. Onemay also look at the instantaneous profile, that is look atinstantaneous power requirement. The system should be designed to ensurethat required power is available. When the instantaneous power usagerequirement is, for example, lower than average power available/powerconverter output, the system may store the excess. When the powerrequirement exceeds the available power, stored energy may be accessed.

Operating one or more processors of the control system to frequently orconstantly monitor energy usage/storage requires significant energy. Indeveloping a strategy to achieve suitable power availability asdescribed above, it is desirable to reduce or minimize the requirementof monitoring by the control system (for example, including one or moremicroprocessors). Using established principles in the electrical arts,energy storage goals, circuit designs, load scheduling etc. may be usedto reduce, minimize or prevent the requirement of monitoring. One may,for example, set targets or set points for different modes of operation,allocating power to storage and/or to load. In a representative mode ofoperation, energy storage setpoints may, for example, be chosen toachieve a certain level of storage and to not drop below that level.

As described above, the second current controller may, for example, becontrolled to draw constant current or to draw current in a slowlyvarying manner (thereby maintaining low noise). The setpoint of thesecond current controller establishes primary current available to thepower converter and affects the charge current (that is, the secondcurrent controller output current and power converter primary inputcurrent) supplying the input capacitance of the power converter which,in turn, determines the magnitude and rate of change of the converterinput voltage. In general, power converters are designed to regulate aset secondary voltage by pulling and converting power available at theconverter input into the secondary current required to maintain thesecondary voltage setpoint. In a number of embodiments of systemshereof, the secondary current pulled from the power converter secondarymay be controlled via a third current controller in electricalconnection with the secondary side of the power converter. Without thethird current controller, the current pulled from the secondary of thepower converter is determined by demands of the load(s). One may, forexample, determine a setpoint or target for the secondary currentdepending upon, for example, the efficiency of power converter and/orthe converter primary voltage and primary current being supplied by thesecond current controller. As described above, a portion of the drawncurrent may be directed to the load(s) and a portion of the drawncurrent may be directed to storage. The difference between the amount ofcurrent pulled through the power converter and that being used by theload(s) may be sent to storage. Without control of the secondary currentpulled through the power converter, the power converter may pull morepower than is available at a given time at its input. It is possiblethat the instantaneous load demand on the converter secondary may notalign with the instantaneous power available at its input in such amanner as to cause the converter to shut down (in the case too muchcurrent is drawn from the primary, resulting in the input voltagefalling below the converter minimum requirement) or failing to captureneeded excess power available at the converter input. Sufficient currentmust be pulled through the power converter to operate the system andloads thereon. One may save power by pulling less than maximum possible.As loading profiles may be known or determined a priori, one may providemodel-based control for developing one or more setpoints for pullingthrough the secondary current.

As illustrated by the storage capacitor charge profile in FIG. 5,predetermined loading schedules or profiles may, for example, be used todetermine the amount of energy storage required. The size of the energystorage system should not be determined in an arbitrarily fashion. Ifthe energy storage system is too large, it may take too long to chargethe energy system to an operating point. It is desirable to choose theone or more elements of the energy storage system (for example,supercapacitors) to be of a sufficient size such that the energy systemmay be charged over reasonable time and all the load requirements may becovered. In general, one may design the energy storage system to coverthe worst-case scenario for an energy storage to load requirement. Onemust provide sufficient capacity to operate the load(s) on the systemand sufficient storage to meet instantaneous load requirement. Asillustrated in FIG. 3B, supplying secondary loads during transitions ofILoop may be an important consideration in planning the charge capacityand charge/discharge profile for the storage capacitor. The amount ofenergy directed to storage may be changed for specific loadrequirements. In general, the system may be designed to deliversufficient power to whatever load(s) will be attached to the system andto include sufficient storage to, for example, meet short-term, highloads.

Problematic situations may arise as the loop current ILoop changes withthe output of the sensor (see FIGS. 6B and 6C). As described above, losscurrents (Iloss) and the current I2 are drawn from the current loop andare returned to the first current controller from all system componentsvia local ground designated GND_local. The loop current may becontrolled as a difference or by measuring ILoop. In the case of controlas a difference, I1=ILoop setpoint−IGND_local whereIGND_local=(12+Iloss). In the case of using direct measurement of Iloop,Iloop_measured−Iloop setpoint=I1 where Iloop_measured is the measuredILoop which includes I1, I2 and Iloss. The second current controllercontrols the margin for I1 in either case. In the case of a fixedexternal voltage supply, when loop current is high, port voltage is low.When current is high on the current loop, and the voltage is low at theport, one will typically have sufficient power at the power converter.However, when the current changes to a low current at the loop port (forexample, in a 3 mA fault situation), there is little available currentto charge capacitors at the input of the power converter to achieve asuitable operational voltage at the power converter input (see FIG. 6A).This problem may be addressed in several manners.

In general, the voltage Vprimary at the primary side of the powerconverter is constrained to be less than port voltage. When the loopcurrent changes from high current to low current, the system mustestablish a suitable Vprimary, but there is little current available todo so (see FIG. 6C). As described above, there is little current tosupply to one or more capacitors at the primary sided of the powerconverter. Energy storage may be used to alleviate such situations. Thesystem may store energy in the higher loop current modes. Stored energymay be used to power one or more loads connected to the system duringlower current modes to provide an opportunity to recharge capacitors atthe primary side of the converter. In that regard, the amount of powerpulled through the power converter may be decreased, while using theenergy storage system on the secondary side of the power converter to(at least partially) power the loads, thereby allowing capacitors on theprimary side to charge on the available current. In a number ofembodiments, extra power may be pulled through the power converterduring high loop current modes to charge the energy storage systemconnected at the power converter secondary. In a number of embodiments,a third current controller (sometimes referred to herein and/or in thefigures as current controller 3) is used to create a load at the powerconverter secondary to pull the additional power from the powerconverter primary to charge the energy storage system connected at thepower converter secondary. In a number of embodiments, the output of thethird current controller is connected to a charge pump that chargesstorage capacitors as illustrated in FIG. 7). In a number ofembodiments, the output of the third current controller may be connecteddirectly to the storage capacitor(s) as illustrated in FIG. 8. Oneskilled in the art will recognize that the switched mode converterand/or switches in the embodiment of FIG. 8 may be substituted withcommercially available integrated circuit super capacitor chargers,monitors and/or supervisors that receive microcontroller instructions tocontrol charging, connection to loads and monitor the state of charge ofthe super capacitors. In low loop current modes, more power may bedischarged from the energy storage system as described above to providea better power conversion mode for the power converter, reducingIprimary drawn into the power converter and allocating more I2 currentto charge the capacitors (Cprim) at the power converter input to attaina target Vprimary (see FIGS. 6A through 6E). FIGS. 6A through 6Eidentify impacts of the second and third current controllers oncapacitor size (capacity) and transient charging of the power converterprimary capacitors. Those figures further illustrate the operation ofthe first and second current controllers to manage the power converterprimary voltage during ILoop transients and the combined action of thethird current controller and load switching to increase and decrease thedemand on Iprimary (through Isecondary) to both capture excess energyand return it to the loads to make optimal use of available power at thepower converter primary and sustain secondary power needs when Pprimaryfalls below PprimaryReq.

In a number of embodiments, energy storage may be provided on theprimary side of the power converter. While loop current is high, thesystem may direct a portion of the current from the secondary currentcontroller to one or more additional capacitors in parallel with the oneor more capacitors at the primary side of the power converter. In lowloop current modes, the additional capacitor(s) may be switched toseries connection, and stored current may be drawn therefrom. Two suchembodiments that capture the limiting current (Ilimit) of a voltagelimiter placed at the output of current controller 2 (to guarantee aminimum required voltage drop ΔVCC2 across the current controller) areillustrated in FIGS. 9 and 10. The voltage limiter sinks current Ilimitwhen ΔVCC2 approaches or falls below the voltage limiter setpoint(Vclamp). This happens most notably when ILoop transitions from low tohigh (and the I2 setpoint is correspondingly transitioned from low tohigh) resulting in a period in which Vlimited is near or below a diodevoltage drop below Vprimary. During this transition, while Vprimary isdischarging to below Vlimited, I2 is shunted to Ilimit and stored inCpstore1 and 2 for the embodiment depicted in FIG. 9 and transferred viathe boost converter from Ccapture to Cpstore1 and 2 for the embodimentof FIG. 10. The charge stored in Cpstore1 and 2 can be used to provideadditional charge current to Cprim to more rapidly raise Vprimay whenIloop transitions from low to high. It may also be possible to feedbackcurrent from secondary side of the power converter. However, suchfeedback would require an inverse converter to push it back to thecorrect voltage on the primary side of the power converter.

As indicated previously, Isecondary loading profiles may be known ordetermined a priori and/or estimated dynamically from measurements madeof associated voltage and/or currents to establish a target Isecondary(ISecTarget) for estimating power required from the external 2-wire loopand the required setpoint targets for the second and/or third currentcontrollers. In a number of embodiments hereof, one may providemodel-based control for developing one or more setpoints for pullingthrough the secondary current. One such embodiment of a model derivedfrom a steady state representation of circuit elements and useful fordeveloping model based control and estimation equations is representedin FIGS. 11A through 11C. For the illustrated model, a steady-stateoperational state is assumed (circuit states in which important circuitparameters such as Vport, Iport, Vprimary, Iprimary, and Isecondary arenot changing) so that transient elements (like the voltage limiter andcapacitors) in the circuit of FIG. 11A are removed and/or replaced withsteady state operational assumptions in the steady state model of FIG.11B. For example, the voltage limiter in FIG. 11A may be replaced by afixed voltage source in FIG. 11B representing a specified marginVclamp_margin that would be required to keep the limiter from sinkingcurrent Ilimit. The steady-state circuit model of FIG. 11B may bereduced further into a simplified model adequate for estimation ofimportant steady-state parameters such as the Vport and Vprimaryavailable from the external loop and the Vport and Vprimary that isrequired to supply a prescribed steady state secondary load current(Isecondary). In this steady state model, all the I2 current fromcurrent controller 2 flows into the power converter primary(Iprimay=I2). Additionally, elements of the first and second currentcontrollers (sometime referred to as current controllers 1 and 2 in theFigures) and current losses prior to the second current controller arecondensed and lumped into model parameters of FIG. 11C. The estimationmodel accommodates variation of power converter efficiency (eta) as partof important parameter estimation.

An example of embodiments of parameter estimation equations derived fromthe steady state model of FIG. 11C are enumerated in FIG. 12. Thesemodel equations are expressed as functions of dependent variableparameters (identified in the square brackets and appended with anunderscore as used in the functional notation convention used in FIG.12). Other variables identified in these equations are presumed to beconstants taken from the model. One skilled in the art will recognizethat these equations may be algebraically rearranged to solve for thedependent variables in terms of the independent function variable andother dependent variables as expressed in FIG. 12. Together, theseequations are useful for estimating limits and targets for setpoints forthe second and/or third current controllers and estimating optimal orobtainable targets for monitored and controlled parameters such asVport, Vinput and Vprimary.

In a number of embodiments hereof, a predetermined secondary loadoperation (profile) is selected and the associated average secondarycurrent target (ISecTarget) for that profile is retrieved from memory bythe microcontroller and/or calculated from composite profile informationfrom memory and/or measured parameters (voltages and/or currents) thatmonitor secondary power (load) usage. The model equations of FIG. 12 canbe used to determine the minimum I2 setpoint (IPrimaryReqMin equation G)and minimum Vprimary requirement (VPrimaryMinReq, equation E) given aselected ILoop setpoint (for current controller 1) and power supplyparameters that must be maintained or exceeded to support ISecTarget.Additionally, equations H (and I if Vport measured at Vinput) can beused to determine the voltage required at the 2-wire port (VportReq). Tomeet the minimum steady state requirements determined from theseequations, the I2 current (IPrimaryMax) available from ILoop at thegiven setpoint for the second controller (IPrimaryCmd; see equation C)must meet or exceed IPrimaryReqMin and the available VPrimary (equationF) and Vport (equation A) must meet or exceed the minimum requirementsVPrimaryMinReq (equation E) and VportReq (equation H). The setpoint forthe second current controller (I2sp) is constrained at every ILoopsetpoint for the first current controller (identified as Iport inequation C) by I2sp=<IPrimaryMax[ILoop setpoint,“Max”] where “Max”indicates that the maximum available current to I2 from ILoop isrequested.

In a number of embodiments, using the combined set of equations setforth above, estimates of the required VportReq at the maximumISecTarget load current ISecTargetMax (where ISecTargetMax is thelargest single or combined ISecTarget of load profiles (thatconcurrently draw Isecondary current) can be estimated using equation HasVportReqMaxLoad=VportReq[ILoop,“Max”,ISecTargetMax,eta_min,VPrimSMPSMin,Vclamp_margin] for ILoop swept across the full range of setpoints(ILoopMin to ILoopMax). In a number of embodiments, the microprocessorcan determine if the external loop will support the prescribedISecTargetMax across the full range of ILoop by assessing thatVportAv[ILoop,Vext,RLoop] (equation A)>=VportReqMaxLoad for all ILoopbetween ILoopMin and ILoopMax. By rearranging terms, equation H can bearranged to solve for the maximum steady state secondary current(ISecTargetMax) that ensures VportReq (equation H)<=available portvoltage VportAv (equation A) available across the full range of ILoop.

As described previously, a problem with a current-fed powerconverter/regulator such as a switch mode power supply powerconverter/regulator is the maintenance of the power converter primarypower (that is, voltage and current) to align with minimal operationalrequirements of the converter (minimum Vprimary) and the power region(Vprimary and Iprimary) where the converter can operate with sufficient(or optimal) efficiency to generate the targeted Isecondary. The minimalrequired voltage that must be maintained at Vprimary for a givensecondary load is related in equation E of FIG. 12. The physicalconstraint in maintaining the primary conditions is tied to thetransient charging the power converter's primary capacitors from thelimited current being driven by the second current controller. Thisconstraint is particularly problematic when there is significant voltageloss on the external loop due to high loop component impedances suchthat the transmitter port voltage changes significantly with changes inILoop. The relationship is modeled simply as Vport=Vext−ILoop*RLoop (seeequation A of FIG. 12) where Vext is the external loop power supplyvoltage and RLoop is the combined resistances of all lossy components(including, significantly, the wire resistance). This equation is in aclassic y−m*x+b form where x is ILoop and the slope m is RLoop, henceVport changes more for a given change in ILoop for high RLoop. Asindicated in the steady state model equations of equation F of FIG. 12,available VPrimary varies directly with Vport, so the change in voltageavailable at the power converter primary increases with increased RLoop,resulting in larger Vprimary transients when ILoop is varied.

Moreover, as indicated in equation F of FIG. 12, the maximum availableVprimary is constrained to be less than Vport so that the maximum limitfor Vprimary is at its lowest when ILoop is high and at its highest whenILoop is at a minimum. Combined with the constraint that I2<ILoop, thismeans that an operational challenge arises when attempting to charge theconverter primary capacitors to increase Vprimary to compensate for thereduction in I2 current to supply Iprimary to maintain the necessary oroptimal primary power to produce the targeted secondary current. Inshort, it is operationally challenging to charge the converter primaryvoltage when ILoop setpoint is decreased.

As described above, the third current controller may be controlled viaits setpoint to pull through excess power available at the powerconverter primary to charge the energy storage capacitors connected atthe power converter secondary. Steady state excess available powerexists at the power converter primary whenVprimary*Iprimary*η/Vsecondary>Isecondary required to sustain thesecondary load. In steady state for a given Iprimary, this occurs whenVprimary>VPrimaryReqMin(@Iprimary, @Isecondary) (see equation E of FIG.12) Additionally, a transient condition frequently exists when ILoop ismodulated from a lower to higher current such that Vprimary at the powerconverter input momentarily exceeds the maximum primary voltageavailable to the converter from Vport (VPrimaryMaxAv see equation F ofFIG. 12). In a number of embodiments that capture excess primary energyin the storage connected to the power converter secondary, thistransient excess power is efficiently captured by increasing thesetpoint of the third current controller momentarily to pull the chargefrom the Cprimary capacitors at the power converter primary to reduceVprimary quickly enough to avoid activating the voltage limiter circuitat the output of the second current controller and to avoid shuntingIprimary to ground via Ilimit (see FIG. 4 and FIG. 6C). Captured andstored energy may then be connected or transferred to the secondaryloads, reducing the demand placed on Isecondary being produced by thepower converter. As discussed previously, reduction of secondary currentdemand from the power converter can be utilized to operate the powerconverter in a mode that draws sufficiently less Iprimary current thancurrent available from I2 to allow the power converter input to becharged to a Vprimary sufficient or optimal to resume delivery ofIsecondary at a rate sufficient to continuously sustain the secondaryloads (see FIGS. 6B, 6C and 6D). In general, the amount of energy storedand supplied from the energy harvester storage must be sufficient enoughto allow the Isecondary demand to be reduced long enough to allowVprimary to be charged to a target level from I2.

As also described above, another practical approach is to store I2current (in excess of the Iprimary required for power converteroperation to sustain the required Isecondary) in capacitors at the powerconverter primary. Among the embodiments realizing this concept arethose depicted in FIGS. 9 and 10. Using the steady state model equationsof FIG. 12, excess I2 current available for primary storage exists, forexample, when the I2 setpoint (I2sp) is commanded such thatIPrimaryMax[ILoop,I2sp] (see equation C of FIG. 12) exceeds the minimumrequired IPrimary (IPrimaryReqMin[Iport,I2sp,ISecTarget,eta,VPrimSMPSMin] (see equation G FIG. 12) As describedabove, primary storage capacitors such as those embodied in FIG. 9 maybe switched from parallel (for excess I2 storage) to series with eachother to create a voltage greater than the target Vprimary to providecurrent to charge Vprimary to a target level as I2 is reduced withreduced ILoop setpoint. In other embodiments, the excess I2 current canbe collected in a capacitor connected in series with a boost converteror charge pump that charges primary capacitors whose output mayconnected to charge Vprimary (see FIG. 10).

The second current controller sets the current pushed to the powerconverter primary in I2 drawn from ILoop and returned to the firstcurrent controller. In addition to controlling noise created in thesystem that is returned to the first controller and ILoop, it is alsopossible to exert some direct control over the way in which the powerconverter operates through manipulation of the setpoint for the secondcurrent controller by using the processor system/microcontroller tocontrol loop current delivered to Iprimary and Vprimary via I2 for agiven average secondary load. With the addition of the third currentcontroller, direct control can also be applied to Isecondary as anadditional measure of influence on power converter operation. One may,for example, indirectly control power converter frequency, rise time,duty cycle etc. to effect power conversion while limiting primary sidenoise by using I2 to manipulate the loop current delivered to Iprimaryand the Vprimary voltage and/or using current controller 3 to directlycontrol the Isecondary load demand presented to the power converter. Useof current the second and third current controllers to influence powerconverter operation through control of converter primary and secondarypower components (voltage and current) enables the potential to exertsuch control on commercially available power converters and controlIC's. Such control is difficult and complicated, however. In general,attempting to limit power converter noise through using control of portvoltages and currents, or through direct control of power converterparameters such as switch frequency, duty cycle, etc., is significantlymore difficult than relying primarily on the use of a high bandwidth,high output impedance, second current controller to contain the primarycurrent noise of a power converter from reaching the current loop.

One may also control the operating parameters of power converter usingthe second and/or third current controllers as just described to controlthe input harmonics. Noise generated by frequency, pulse width, risetime may be controlled to a certain degree by the operation of theconverter within a predetermined range of operating conditions. In thatregard, one may, for example, control Iprimary (and thereby controlVprimary) and control Isecondary in a manner to reduce noise produced bythe power converter. Such control of the operating conditions of thepower converter may be used in conjunction with the high-bandwidth, highoutput impedance, second current controller to restrict noise on thecurrent loop. Control of the setpoint of the second current controller(to control 12), in conjunction with control of the setpoint of thethird current controller (to control the current pulled through to thesecondary side of the power converter) may be used to keep the powerconverter in a desirable or optimal operating condition. Providingcontrol of the region of operation of the power converter may, forexample, be advantageous to restrict noise produced by the powerconverter to be within a determined bandwidth for the second currentcontroller.

As described above, one can optimize Iprimary (and thereby Vprimary) foroperation of the power converter. In embodiments with a second currentcontroller, the second current controller controls Iprimary (and therebyVprimary at a given secondary load current Isecondary). Use of thesetpoint for I2 via the second current controller to control Iprimarymay, for example, be referred to as a push methodology (see, forexample, FIG. 2B and FIG. 13). In embodiments wherein the third currentcontroller is present, the current pulled through the power converter,Isecondary, is controlled by the third current controller (see, forexample, FIG. 2C). Use of the setpoint for the third current controllerto control Isecondary may, for example, be referred to as a pullmethodology (see for example, FIG. 14). The push control methodologycontrols Iprimary and Vprimary (for a given Isecondary) only.Incorporation of a pull methodology via the third current controller onthe secondary side of the power converter additionally enables one topull Isecondary to be a certain level or setpoint. Vsecondary is afunction of the power converter setting. The operations of the secondand third current controllers can be combined/synchronized tosimultaneously capture maximum power from the external loop and deliverit to the secondary. Such combined/synchronized operation of the secondand third current controllers may, for example, be referred to as thepush-pull methodology (see FIG. 15).

In control methodologies hereof, sufficient Vprimary is required so thatpower converter doesn't shut off. If Vprimary drops below theconverter's minimum input voltage threshold, the power converter shutsoff. If Vprimary cycles above and below the minimum Vprimary thresholdrequired for operation, the power converter will turn on and off. Asdescribed, above, control methodologies (for example, using model-basedcontrol) hereof control Iprimary to maintain Vprimary at or above thevoltage required to supply Isecondary (see equation E of FIG. 12). Withknowledge of the secondary power requirements and power converterefficiency (η), such models may, for example, control Vprimary and steerit within the bounds VPrimaryReq (see equation E of FIG.12)<=Vprimary<=VPramaryMaxAv (see equation F of FIG. 12) to be optimizedfor the amount of current available.

Power control to one or more loads on the secondary side of the powerconverter may also be utilized. As described above, the energy storagesystem may be used to maintain a reserve of energy with knowledge ofwhat power requirements will be for one or more given load and/or modeof operation. One may further control one or more of the loads to, forexample, decrease the power usage thereof and or to set a time periodfor the power usage thereof. Devices, systems and methods hereof may,for example, allocate power to one or more loads based on modes ofoperation. For example, power to one or more loads other than the sensormay be reduced to allocate power to the sensor. Likewise, certainperipherals may be placed in a lower power or zero power state toallocate power to the sensor.

In determining the control of various aspects of the devices, systemsand methods hereof, the control system may, for example, monitor variousaspects of the system such as the power available at the port, I2,Vinput, Iprimary, Vprimary, Isecondary etc. In a number of embodiments,variables such as the power at the port may be measured and setpoints asdescribed herein may be set based on the measured power etc. In a numberof embodiments, one may adjust energy harvesting to harvest all energythat is available for a given port power.

In a number of embodiments, the third current controller determines thesetpoint for Isecondary and allocates the portion of Isecondary that isnot consumed by loads directly connected to the secondary (loads notdrawn from secondary storage) to be sent to the energy harvesting systemenergy storage subsystem (see FIG. 2C). The setpoint for the thirdcurrent controller may, for example, be driven in a number of aspects.In a first aspect, the processor system may set a minimum thirdcontroller setpoint to guarantee an excess of Isecondary is available tosupply the minimum amount of current required to maintain the minimumstate of charge of the energy harvester storage system to sustain thesecondary operations. In a number of embodiments, where the output ofthe third current controller is connected to drive current to thestorage capacitor(s), this is implemented by setting the third currentcontroller setpoint at or above the average load current that factors inthe secondary storage capacitor charge profile in the average loadcurrent for the a predefined load profile (see dashed line representingthe average profile period current and composite secondary loads thatinclude the storage capacitor charge profile in FIG. 5). In this case,the average load current associated with the profile factors in thesecondary loads drawn dynamically directly from the Isecondary and/orfrom the storage system with the charge current required to restore thestate of charge of the storage system to the level required at the startof the profile period ΔT (see the storage capacitor charge profile inFIG. 5). In a second aspect, Isecondary may be maintained above someminimal level by issuing an Isecondary setpoint that is compared to adynamic measurement of the portion of Isecondary that is actually beingpulled by the combined secondary loads (including by the third currentcontroller to the energy storage system) and increasing the setpoint forthe third current controller above the level described in the firstaspect described above to be equal to the difference in Isecondarysetpoint with the measured Isecondary. In that regard, in a number ofembodiments, the third current controller setpoint is set to be equalto: minimum setpoint+Isecondary setpoint−Isecondary with the constraintthat the secondary term is limited to Isecondary setpoint≥Isecondary.

In a number of embodiments, the Isecondary setpoint serves to levelIsecondary to limit the extent that that the power converter is requiredto change operation in response to secondary load transients. Changes inpower converter operation can introduce transients on Iprimary. Toprevent these Iprimary transients from propagating to ILoop, they shouldfall within the combined bandwidth of the second current controller andthe frequency region where passive components connected between thesecondary current controller and the power converter input are capableof supplying or suppressing the transients passed to ILoop below thetargeted noise level. Another control aspect providing in setting of theIsecondary setpoint is to maximize the conversion and storage ofinstantaneous current that is available at the power converter inputinto secondary current that can be captured and stored for use to supplyloads that require short term power in excess of the nominalinstantaneous power. In a number of embodiments, setting of theIsecondary setpoint thus serves two important functions: (i) limitingthe operation range of the power converter (for example, a switch modepower supply) to keep Iprimary noise to within targeted levels and (ii)to allow maximal or optimal conversion and capture of power available atthe power converter input to sustain secondary operations.

Certain design considerations of a system hereof are discussed inconnection with representative case studies of 4-20 mA system hereof.Some important parameters and considerations in the design of the 2-wiretransmitter 4-20 mA port specification are first discussed. The minimumVport voltage required to power the instrument when ILoop is at itsmaximum value. This is commonly referred to as the compliance voltage orsometimes referred to as the lift-off voltage. The instrument compliancevoltage imposes constraints on the voltage of the external loop supply,the maximum measurement load and the length of the wire connecting theseto the instrument. The compliance voltage is typically selected toaccommodate a target wire length (wire resistance) with industrystandard loop power supply voltages and measurement load resistorvalues. Conversely, the compliance voltage may be chosen to provide therequired power for selected instrument features with specifiedconstraints for the loop power supply and combined wire and measurementloads. The minimum and maximum ILoop current are also importantconsiderations. The minimum ILoop current is typically specified to somevalue below 4 mA to indicate a fault condition for the instrument. Themaximum ILoop current is typically specified to some value above 20 mAto indicate a measurement over-range or lock-out condition. The maximum4-20 mA loop current resolution is maximum number of discrete levelsbetween 4 and 20 mA required to represent the minimum measurementinterval across the full-scale range. This specification limits theminimum ILoop step change the transmitter must resolve and thus limitsthe maximum peak-to-peak current noise the transmitter can impose onILoop. As described above, the peak-to-peak noise may, for example, beless than ½ the required minimum increment of ILoop to avoid unwantedfluctuations (flicker) when the loop current measurement is quantized.As also described above, inclusion of a wired HART modem requires a+/−0.5 mA modulation depth, adding overhead that limits the portion ofILoop that can be used to power the instrument. Additionally, since netwire capacitance is a function of line length, the line impedance limitimposed by the 2.4 kHz max HART FSK (frequency shift keying) signalingfrequency limits the maximum loop distance.

The specification of the port capabilities for the transmitter hereofbears consideration of the power that may be expected to be availablefrom the 4-20 mA loops in existing installations. When used as areplacement transmitter, the new transmitter design should ideally becapable of delivering functionality equivalent to or better than thetransmitter it is replacing with minimal or no requirement foralteration of the existing loop components. The capabilities of existingtransmitters may be used as benchmarks to gauge modifications in portrequirements driven by the need to power additional features and highercurrent loads in the new transmitter designs hereof.

General requirements for a number of embodiments of a 2-wire port andexternal 4-20 mA loop are outlined below. Requirements that are believedto be known as well as those that must be determined for a particularsystem are discussed based, for example, upon projection of powerestimated for features proposed for the new energy harvesting platformto the transmitter port. An ILoop minimum of 3.5 mA max (that is, afault condition) may, for example, be specified. It is desirable thatILoop minimum be as high as possible to keep minimal available power atthe port as high as possible. In a number of embodiments, ILoop maximumrequirement is 22 mA min (that is, an over-range condition). The ILoopresolution requirement is a maximum of 1000 steps across full scale 4 to20 mA range or 16 uA minimum resolution. The ILoop noise requirement nogreater than 8 uA (peak to peak). The maximum Vport is 30 VDC (but thatmay be increased in certain embodiments).

As described above, the compliance voltage requirement is the minimumVport at maximum ILoop. The compliance voltage is dependent on thesecondary load(s) the instrument must support. Specific requirements forcompliance voltage will be discussed further below. The compliancevoltage required to support the secondary current load drawn bydifferent types of sensors and various combinations of optional featuresmay vary significantly. Increasing the secondary load requires anincrease in the compliance voltage which, in turn, impacts the minimumloop power supply voltage and combined wire length and measurement loadresistance that can be supported on a 2-wire loop. As a result, thevalue of functionality afforded by optional features/loads must beweighed against associated impacts to loop capabilities. Rather thanindicating a single compliance voltage, a matrix of compliance voltagescorresponding to sensor type and supported optional feature combinationsmay, for example, be specified.

External loop component constraints must also be considered. A maximumsupply voltage should be equal to the maximum Vport specification. Thehistorical maximum voltage for a number 2-wire sensor systems has been28 VDC or 30 VDC. Higher loop voltages (and Vport maximum voltagecapability) may be considered as a means to enable longer loop distancesor to accommodate fixed loop sense resistances while supporting highercompliance voltages necessary to supply combined power requirements forfeatures/loads installed on a 2-wire transmitter. A minimum supplyvoltage will be dependent on the combined power requirement for thefeatures/loads installed on the 2-wire transmitter.

Further considerations include loop wire resistance and measurementload. Loop wiring for a 2-wire installation will often be preexisting.Thus, the design should accommodate typical minimum wire gaugeshistorically used for such applications. In a number of installations,the minimum wire gauges have been 22 AWG (16 mΩ/ft) or 18 AWG (6 mΩ/ft).In a new wire installation, larger wire gauges may be used to supportlonger loop lengths with standard measurement resistances. For thisreason, the 2-wire platform may, for example, accommodate a maximum wiregauge of at least 14 AWG (3 mΩ/ft). While 500Ω measurement loads havebeen commonly used to achieve a convenient 2 to 10V measurement rangeacross 4 to 20 mA, the added port compliance voltage required to powerfeatures on the platform hereof may significantly limit the loop lengthwith a measurement load this large with a standard 24 VDC loop supply. A250Ω or smaller measurement resistor may be required to accommodate thelength and wire gauge of an existing installation with a 24 VDC loopsupply.

In the case of replacing an existing transmitter, the transmittershereof should be able to deliver at least equivalent functionality andgas monitoring capability without modification of existing loopcomponents to serve as a drop-in field replacement. The capabilities ofthe 4-20 mA port should thus meet or improve on such legacytransmitters. In consideration of supporting new features andcapabilities beyond those of legacy transmitters, additional power maybe required from the external loop, requiring either an increase in theloop voltage supplied and/or a reduction of the loop measurementresistance. The capability introduced by the replacement of the linearpower supplies used in legacy transmitters with a power converter insystems hereof introduces a new paradigm allowing use of elevated portcompliance voltage to acquire added power to support new sensors andfeatures. A transmitter port hereof may, for example, be designed toaccommodate loop supply voltages beyond the 30 VDC limit supported by anumber of legacy systems to facilitate a practical means of using theloop power supply to reconcile existing wire length and measurementloads with the reduced loop voltage margin imposed by elevatedcompliance voltages.

Block diagrams illustrating embodiments of a proposed power supplysystem solution for a 2-wire platform hereof are set forth in FIGS. 2Cand 2D. Those figures are partitioned into two major sections, a first,leftmost section representing the 4-20 mA loop components external tothe transmitter and a second section, positioned right of the firstsection in the Figures, setting forth a high-level representation of thetransmitter signaling, energy harvesting and load scheduling features.The primary loop control subsystem and the energy harvester system asdescribed above. As described above, the energy harvester subsystem isprincipally responsible for supplying the average and peak power for thecombined transmitter loads from the available P2 power (wherein(P2=V2*I2) and stored energy. The energy harvester subsystem includesthe power conversion system and the energy storage/retrieval system. Ina number of embodiments, the power conversion system implements amaximum power point tracking MPPT control that maximizes the conversionof available input power (P2=V2*I2) into secondary current Isecondary orIs at a prescribed secondary voltage Vs. In a number of embodiments, thepower conversion system implements control to attain and sustainconditions to affect voltage and current parameters of the system tominimize noise returned to the external loop while maintaining adequatesecondary supply and storage to sustain the secondary load profilescomprising the current mode of operation. This subsystem is responsiblefor supplying the average power consumed by the transmitter loads. Theenergy storage/retrieval system stores the residue of the instantaneousexcess secondary power not consumed by the transmitter loads and returnsit to supply peak demands in excess of the average secondary powerproduced by the power conversion system.

Referring primarily to FIG. 2D, a third component of the power controlsystem is a load control subsystem that manages the timing and/ormagnitude of peak load demands IL to avoid overwhelming the combinedaverage current supplied by the power conversion system Is and the shortterm storage current I_(ST) drawn from the finite energy storage system.The rectangular boxes set forth in, for example, FIG. 2D, within theload and load control subsystem block, illustrate classes of secondarytransmitter loads that are to be accommodated by the power controlsystem. Those classes of loads are described briefly below.

Constant Loads represent the composite static DC quiescent current drawnby all loads connected to the transmitter secondary. Constant loads donot vary appreciably over time and thus do not rely on instantaneouspeak power provided from the energy storage system. However, the steadystate nature of these loads requires they be allocated directly from thesecondary power produced by the conversion system.

Random Autonomous Loads represent peak loads that can occur randomly andcannot be anticipated, monitored or controlled. Accommodating thecombined random peak currents of such loads requires a reserve beallocated in the energy storage to service them. Since these loadscannot be anticipated, the control system cannot schedule loads it candirectly control to avoid simultaneous peak demands. Energy storagemust, therefore, be allocated to accommodate the eventuality of the peakpower demands of all random autonomous loads aligning with each other orwith schedulable loads. This requirement limits the flexibility ofcontrol system to distribute peak power and limits the features/loadsthat can be supported. In a number of embodiments, random autonomousloads may be avoided—for example, by either modifying the design torender them predictable or controllable or by limiting the peak currentof loads that cannot be modified to preserve energy storage margin.

Periodic/Predictable Autonomous Loads represent peak loads that, whilenot directly controllable, occur in a regular interval that can bemonitored and predicted by the control system. This type of load ispreferable to random autonomous loads since the timing of peak loadsdirectly enabled by the control system can be scheduled to avoid orlimit excessive simultaneous peak demands. While the predictable qualityof the peak timing improves the power allocation options of the controlsystem, energy storage reserve must be allocated to accommodate theeventuality of alignment of the maximum peak demands of the combinedautonomous loads attached to the transmitter secondary, limiting theavailable capacity to support additional features/loads.

Schedulable Loads represent peak loads with a fixed or preset peak powertiming profile that can be enabled and disabled by the power controlsystem. While the peak power profile is fixed, the ability to schedulethe timing of this type of load facilitates controlled avoidance ofexcessive peak alignment with other loads and increases the flexibilityin sharing stored energy to power a broader range of features andfunctionality.

Adjustable Schedulable Loads represent loads with adjustable powerprofiles (adjustable peak magnitude and timing) that can be adapted torealize performance vs peak power tradeoffs to enable features to beoffered at the best performance level (or at least a minimallyacceptable performance level) while accommodating the available powerand power sharing demands of other features. Loads with this qualityafford the greatest flexibility to the control system to power thebroadest range of features. To facilitate the greatest range of featuresand adaptability to field power availability, all high peak demand loadsand optional loads may be designed to accommodate adjustable loadscheduling.

The above classifications of loads set forth control accessconsiderations important to successfully managing multiple peak powerloads in a system powered by an energy harvesting based power supplysystem. A detailed system design emphasizing forward planning tofacilitate load requirements is important in both assessing feasibilityand implementing the design of features with peak load demands.

The power monitoring and load control features of embodiments of thepower supply systems facilitate a number of functionalities. Forexample, the power port monitoring feature, in concert with the primaryloop control, may be used to implement an automated assessment of theexternal loop power supply voltage and port load. This may includemeasurement of the port voltage with the loop current command set to twodistinct values. This functionality may, for example, be used atinstallation to assess if the power supply and load connected to thetransmitter port will support the power needs of the features connectedor enabled on the transmitter. The information may be used to adapt theperformance level of select features to the highest supported by thepower available at the port. Load scheduling and control may also beused to implement operational modalities in which the power requirementand performance level of select features/loads can be modified toprioritize the core functionality of the modality. In other word, thismethodology entails modifying the performance level of select featuresto lower their power demand to make more power available to afeature/load or set of features/loads central to the focus of currenttransmitter operations. As an example, the advertising rate of BLUETOOTHcommunications may be reduced to make more power available to a localuser interface when a user is interacting with the transmitter throughthe local user interface.

General operation procedures may be categorized as initialization(initial power-up or perhaps reboot) and general operation procedures,which covers transmitter power-up through transition to normaloperation. Some of such procedures may, for example, be initiated and/oradjusted under user control in addition to/or instead of automaticinitiation.

In the earliest phase of a transmitter power-up sequence, following abrief in-rush, the transmitter may, for example, initiate with an ILoopcurrent that is less than the normal operational fault current. Anobject of this operational phase is to power and initialize themicroprocessor and other circuits necessary to loop current and powersystem control. Schedulable and adjustable loads may, for example, bedisabled or set to minimum power during this preliminary sequence.

The processor system (including, a microprocessor in the illustratedembodiment) may, for example, initiate an assessment of the availablepower from the 4-20 mA port by systematically adjusting ILoop to twodistinct setpoints and measuring Vport. The ILoop setpoints used forthis port interrogation may, for example, reside below the 4 mAoperational level. Since Vport=Vext−ILoop*RLoop (see equation A of FIG.12) where Vext is the external loop power supply and RLoop representsthe combined port load of the measurement resistor and wire resistance,both Vext and RLoop can be determined algebraically from two ILoopconditions as two equations with two unknowns. This operation may becarried out automatically at first power-up of the transmitter. If it ispreferred that this assessment not be made at every power-up or reboot,this operation could be transferred to a user initiated processthereafter by setting a non-volatile flag denoting the first assessmenthas been performed.

Depending on power availability assessed at the port, the transmittermay need to institute a secondary storage charging cycle beforeproceeding. During this cycle, ILoop may be held at a fault level andschedulable and/or controllable secondary loads may be halted or set forminimal power to allocate maximum secondary current to charge thestorage device. This initial ILoop may be implanted for example in anumber of embodiments by providing a constant steering current sourceIREF with the set-point for current controllers 1 and 2 set to 0 (seefor example, FIG. 11A) Alternatively, the charging cycle could beinstituted after assessing the secondary load inventory or performed inthe background once normal operation begins with non-criticalschedulable loads utilizing storage brought online once storage reachesa predetermined acceptable level.

The microprocessor may also take inventory of the loads connected to thetransmitter secondary. Load information may be acquired from constantsstored in memory, through interrogation of sensors and circuits and/orby assessment made from measurements drawn from the power monitoringcircuits flanking the switch mode power supply. Through thisinterrogation or stored configuration settings, the microprocessor maycategorize the loads into classes organized by priority and schedule oradjustment control capability.

The microprocessor may then perform an assessment to determine ifsufficient power can be acquired from the connected 4-20 mA loop tosupply the transmitter loads assessed and inventoried in the previousstep. One approach to performing this assessment is to project thesecondary loads to the port via a port power estimation model (asdiscussed further below; see FIG. 16A) and compare directly to theavailable port power determined at power-up. Conversely, the availableport power may be projected to the secondary and a comparison performedwith secondary current values. Actions taken based on this assessmentwould depend on the level of sophistication deemed appropriate orvaluable for the system. In a simple case, if available port power isless than the power required to run all the secondary loads at a fixedperformance level, a fault state may be entered. Otherwise systemoperation may proceed without entering a fault state. In a more complexsystem, possessing the ability to adjust the operation of some featuresto tailor performance to the available secondary current, other outcomesare possible. For example, if the available port power is less than thepower required by the sum of all secondary loads required to achieveminimal acceptable transmitter capability with all adjustable featuresconfigured for minimum power, a fault state may be entered. If theavailable port power exceeds this minimum, one may proceed to anoperational state using some simple rules. For example, the performanceof select adjustable features/loads may be modified to realizeacceptable or optimal performance supported by the available power.Another possibility is to enable or disable select non-critical featuresas an alternative manner of tailoring the secondary load to conform tothe available port power. In this case, a warning (non-fault) may bedisplayed or communicated to indicate features are disabled. Once powerassessment is completed and/or the system is adjusted for bestperformance with the available port power, typical transmitteroperations may begin to bring the system to an operational state withthe port current (ILoop) transitioned into the normal 4-20 mA range.

During an operational state, and as discussed above, load scheduling andload adjustment may be used to implement operational modalities in whichthe performance of select features can me modified to prioritize thecore function of a specific modality. A principle employable in suchcases is to enable/disable or modify the performance of selectnon-critical features (for example, not critical to the principal gasmonitoring task in the case of a gas sensor) to allocate power tofeatures central to a current modality or mode of operation. In otherwords, this methodology utilizes a redistribution of available power toprioritize features associated with a preferred focus of transmitteroperation.

A number of embodiments of automated transmitter capability assessment(determining transmitter operational capability from available portpower) discussed above are described in further detail. The discussionbelow begins with separate treatments of port power availabilityassessment and requirement assessment and concludes with transmitteroperational capability determination, which combines those assessments.

The objective of port power availability assessment is to estimate thesteady-state power delivery capability of the external 4-20 mA loop tothe transmitter port. More specifically, this involves assessment of thesteady-state open loop voltage VportAv range available over thetransmitter's full operational ILoop current range. To facilitateestimation of Vport_(A) at arbitrary values of ILoop, a simple modelreducing the external loop to a supply voltage Vext and consolidatingthe loop measurement resistor Rmeasure and total wire resistance Rwireinto a single resistor RLoop is employed as depicted in FIG. 16B. Thesteady-state, open-loop voltage equation at the port is taken from thismodel as: Vport_(Av)=Vext−ILoop*RLoop (see equation A of FIG. 12). Onceagain, this equation takes the familiar y=mx+b linear form wherein theslope m in this case is −RLoop and the intercept b is Vext. As describedabove, the transmitter performs a loop interrogation by setting ILoop totwo distinct setpoints (ILoop1 and ILoop2), and measuring Vport_(A)associated with each setpoint, which produces two equations with twounknowns (Vext and RLoop) having a simple algebraic solution:

Vport_(Av1)=Vext−ILoop1*RLoop; and

Vport_(Av2)=Vext−ILoop2*RLoop.

An advantage of using this approach is that the model equation may beused to calculate the steady-state voltage Vport_(A) across the entire4-20 mA operating range of the transmitter based on data collected atonly two ILoop setpoints. The setpoints chosen for test may bedetermined for a particular system but typically should fall below thelower 4 mA bound of the normal loop operating range. There are a numberof possibilities to consider regarding how and when such a loopinterrogation should be initiated. Such an interrogation may, forexample, be initiated automatically at initial power-up (commissioning),at any power-up or main processor reboot, and/or under user control. Ingeneral, the transmitter typically minimizes internal loads that may becontrolled during the interrogation process. Once Vext and RLoop areestimated, the steady state port voltage may be projected across thetransmitter's full control range for ILoop by simply calculatingVport_(A) vs ILoop as illustrated in FIG. 16C. These projections ofVportAv vs ILoop represent load lines that indicate both the range ofVportAv and the rate of change in VportAv as ILoop is varied across thefull scale range. Since, in steady state, Vprimary is constrained to beless than Vport, the intercepts of VportAv at the minimum and maximumILoop regions that the system must operate limit the primary voltageVprimary that is available to supply the secondary loads that must besupplied across the full scale range of ILoop. Additionally, sinceVprimary<VportAv (see VPrimaryMaxAv equation F of FIG. 12), the slope ofVportAv vs ILoop dictates the steady state change in Vprimary requiredas ILoop changes. The rate or slope of this change In Vprimary is ofimportance in the dynamic control design that issues setpoint commandsto the second and/or third current controllers as it impacts the rate atwhich these current controllers must make adjustments to achieve a newVprimary target as ILoop changes. FIG. 16C depicts the VportAv vs ILoopcharacteristic for three different external 4-20 mA loops of differentVext and RLoop. Interrogation setpoints are marked by an x in FIG. 16C.Dashed lines separate a normal operating region (center) from regions onleft and right representing, respectively, fault and overrange regions.The VportAv vs ILoop values calculated by the fitted model of FIG. 16Crepresent the max voltage available to the transmitter vs ILoopsetpoint. As illustrated in FIG. 16C, the slope of the VportAv lineincreases (negatively) for higher RLoop and determines the range ofchange in VportAv (VportAv@ILoopmin−VportAv@ILoopmax) across the ILoopoperating range [ILoopmin, ILoopmax] and the external loop power supplyVext determines the offset for VportAv across this range in ILoop.

The objective of port requirement assessment is to estimate thesteady-state port power (VportReq vs ILoop) that must be available tothe transmitter for operation. This determination involves takinginventory of the secondary loads (loads attached to the output of thepower converter) connected to the transmitter, summing those loads todetermine the continuous average power demand, and then projecting thisdemand to the transmitter port to establish the required port power. Theprocess begins with the processor system/microprocessor taking inventoryof the secondary loads connected to the transmitter. As described above,load information may be acquired from constants stored in memory,through interrogation of sensors and circuits and/or by assessment madefrom measurements drawn from power monitoring circuits flanking theswitch mode power supply.

In keeping with the load classifications in the load control subsystemdescribed in connection with FIG. 2D, loads that cannot be adjusted orscheduled may possess a single load requirement while loads that can beadjusted and/or scheduled may possess multiple discrete average loadrequirements depending on configuration and scheduling rate. Forsimplicity and robustness, load estimates for loads that may becontrolled by the processor system/microprocessor may, for example, beprecompiled into a lookup table. The details and procedures regardingadaptable load control for a specific system embodiment may be readilydetermined. A configuration defining a minimal acceptable transmittercapability may, for example, be instituted to establish a correspondingminimal power requirement. The secondary loads discovered in theinventory process may be summed together to determine an averagesecondary load.

Projection of the average secondary load to the transmitter port isaccomplished using the model equations from the port power estimationmodel of FIG. 16A. As discussed previously, the origin and derivation ofthe steady state estimation model in FIG. 16A is illustrated in FIGS.11A through 11C with the associated model steady state model equationsenumerated in FIG. 12. These are simple algebraic equations that accountfor the losses in the primary loop control circuits and account for thevariability in the power converter primary voltage VPrimary requirementswith varying primary current IPrimary (that is limited by ILoop) and thesecondary load. Using the model of FIG. 16A, the total average secondaryload is projected to the transmitter port producing a port voltagerequirement VportReq (see equation H of FIG. 12) dependent on the ILoopsetpoint. FIG. 16D illustrates a 4 mA average secondary load projectedto the transmitter port for three different switch-mode power supply(SMPS) configurations (a first Buck converter configuration, a secondBuck converter configuration and a single-ended primary-inductorconverter or SEPIC, which are described in further detail below).

An operational capability assessment may, for example, compare the powerrequired at the transmitter port for given secondary load condition ormode of operation to the power available at the transmitter port. Insimplest terms, this analysis step simply determines if the voltageavailable to the transmitter port VportAv (equation A of FIG. 12)exceeds the VportReq (equation H of FIG. 12) over the transmitter's fulloperational range of ILoop. This concept is illustrated graphically inFIG. 16E wherein VportReq to sustain a 4 mA secondary load with a buckconverter is plotted against the available port voltage VportAv for twodifferent exterior loop embodiments. Both exterior loops include a 24Vloop supply. However, one exterior loop has a loop load resistance of500 ohms, and the other has loop resistance of 800 ohms. As illustrated,sufficient voltage will be available to the transmitter connected to theloop with 500 ohms resistance as the VportReq remains below theavailable VportAv across the entire operating ILoop range of thetransmitter. The loop with 800 ohm resistance (black) is incapable ofsupplying sufficient power to the transmitter for full range operationas the corresponding VportAv load line falls below the VportReq line forILoop>15 mA (wherein the intersection of the lines at 15 mA is markedwith an x).

Since the transmitter performs the assessments of VportAv whileoperating in the fault region, operational capability assessment canproceed as long as the VportAv vs ILoop characteristic of the loopremains above VportReq in this fault region of ILoop. The ILoopintersection ILoop of VportAv and VportReq is defined whereVportAv=VportReq, so that, in practice, capability assessment may berealized by numerically finding the ILoopx at which this condition ismet and determining whether it falls outside the operational ILooprange. For this capability assessment, the IPrimaryCmd term in theVportReq equation (equation H FIG. 12) is set to “Max” (see equation Cof FIG. 12) In the simplest assessment, the transmitter may remain in afault mode and possibly display a condition indicating insufficientavailable loop power for full operation.

As described above, operational modes may be extended beyond basic “goor no-go” decision at power-up to applications in which the operationalcapabilities of select transmitter subsystems can be tailored(maximized) to operate within the available power limit of the loop.This operational mode may, for example, include an ordered adjustment ofselect transmitter subsystems (with scheduling and/or adjustmentcapabilities) with the goal of tailoring VportReq to provide levels ofperformance dependent on the VportAv available to the transmitter. Suchan operational decision mode may, for example, include simple selectionamong discrete configurations pre-compiled into a look-up table. Eachconfiguration in this lookup table arrangement may specify a loadexpected when operating in the given configuration. The minimalcompliment of configurations available in the table would include apower-up configuration and a minimum acceptable operational performancelevel for the transmitter. A power-up configuration minimizing thetransmitter power requirement during start-up would be useful to lowerVportReq as low as possible to permit capability assessment to becarried out with the minimal VportAv over the fault current region. Theminimal acceptable operational performance configuration represents theminimum VportReq across the entire ILoop operating range and hence, theminimal VportReq defining a “go or no-go” condition. Otherconfigurations might define transmitter load requirements forperformance improvements of select subsystems. An operational capabilityanalysis with multiple configurations may be performed. A configurationmay be selected which is the highest performance configuration that canbe supplied by the loop. Transmitter operational capability assessmentfacilitates automated determination of the external loops capability tosupport defined load configurations of the transmitter. This capabilityaffords opportunities for multiple applications ranging from simplefault detection to potential implementation of more advanced performancevs power optimization schemas.

The above considerations and requirements are discussed further below interms of partitioning in a transmitter system between “base” transmitterload requirements and an enumeration of options and proposedfeatures/loads that incrementally increase that base load. Such apartitioning is illustrated in FIG. 2E. The partition labeled A in FIG.2E may be represented by the port power estimation model illustrated inFIG. 16A that represents the associated port current control and powerconversion block. As discussed above, this model is represented by a setof system equations that may be used to project the base and secondaryloads connected to the transmitter through the power conversion stage tothe transmitter port (equations A through I of FIG. 12). Once again,this model also allows examination of the impacts of power converterefficiency (η or “eta”) and primary voltage limits as those variablesare projected to the transmitter port. Conversely, the power madeavailable by the external loop may be projected through these equationsto constraints on the power converter and the transmitter secondaryload. Power losses from the port to the transmitter loads are alsoencapsulated in the model. In addition to its usefulness in estimation,the model of FIG. 16A may also be used as part of a model-predictivecontrol regulating the power converter primary (IPrimary and VPrimary)to maximize the secondary current (ISecondary) produced from the portpower at the loop current setpoint (ILoop). As an example, in a numberof embodiments, a maximum available Isecondary can becalculated/predicted by finding the ISecTarget that maximizes the powerconverter primary power PPrimMax=VPrimaryMaxAv*IPrimaryReqMin for agiven power converter efficiency (η) with the IPrimaryCmd set to “Max”and the Vport argument in the VPrimaryMaxAv equation set to VportAv (seeequations C, G and A of FIG. 12).

As described above, the purpose of the SMPS is to convert a portion ofthe power available at the transmitter port Pport to secondary currentISecondary to supply internal transmitter circuitry and sensor loads. Ageneric SMPS model is, for example, shown in context connected to thetransmitter port and external 4-20 mA loop in the port power estimationmodel of FIG. 16A. Characteristics for important parameters identifiedin FIG. 1A are illustrated graphically in FIGS. 17A, 17B, 17C and 17D.The case represented in those figure results from the followingconditions: Vext (external loop supply) is 24V, Rloop is 500 ohmmeasuring resistor plus 1000 ft of 22 AWG wire (total 516Ω), thesecondary load (ISecondary) is 4 mA, the secondary voltage (VSecondary)is 4.5V and a Buck converter SMPS is use at 90% efficiency (η).

Referring to the parameters identified in, for example, FIG. 16A, thegeneral equation for secondary current for the may be expressed as:

ISecondary=VPrimary*IPrimary*η/VSecondary

VPrimary is the converter primary voltage. Limits associated withVPrimary are depicted in FIG. 17A. The maximum boundary imposed uponVPrimary is VPrimMax, as modeled in equation F of FIG. 12 where,specifically, Vport is set to VportAv (equation A of FIG. 12), andIPrimaryCmd is set to “Max”. VPrimMax is constrained to be less than thetransmitter port voltage Vport. As discussed previously, the steadystate available port voltage VportAV is determined by equation A of FIG.12. To deliver the required ISecondary with a given VSecondary, IPrimaryand efficiency η, VPrimary must exceed the limit defined by VPrimReq asfollows:

VPrimary≥VPrimReq=VSecondary*ISecondary/(IPrimary*η)

VPrimary is also limited by the lowest voltage at which the convertercan operate and still supply VSecondary. Since VPrimary is less thanVport, this minimum VPrimary limit (designated VPrimMin herein andidentified as VPrimSMPSMin in the equations of FIG. 12) for the SMPS canproject limitations on the minimum VportReq. The governing steady stateequation for VPortReq is specified in equation H of FIG. 12. Foroperation VPrimary≥VPrimReq≥VPrimMin. Since Vport>VPrimary, the requiredSMPS primary voltage needed to sustain the secondary load demandVPrimReq and the SMPS minimum primary voltage VPrimMin impose minimumbounds VportReq on Vport to sustain transmitter operation across thefull range of ILoop as follows:

Vport≥VportReq>VPrimary≥VPrimReq≥VPrimMin.

As indicated in FIG. 17A, for a given port supply voltage and loopresistance, the efficiency of the power converter is typically the mostlimiting factor at low ILoop while the power converter minimum primaryvoltage (VPrimMin) tends to be most limiting at high loop currents.

As set forth above, IPrimary is the converter primary current. Limitsassociated with IPrimary are depicted in FIG. 14B. The maximum IPrimary(IPrimMax) is limited by losses in the transmitter to a value less thanthe transmitter port current Iport (ILoop) as modeled in the steadystate equation C of FIG. 12. To sustain SMPS operations for a givenISecondary load current, IPrimary must be kept at or above a minimumcurrent IPrimReq (see IPrimaryReqMin equation G of FIG. 12). As also setforth above, VSecondary is the converter secondary voltage. This voltageis typically fixed at design to a voltage sufficient to meet or exceedthe voltage overhead of all the secondary loads. Once again, η is theconversion efficiency, and accounts for the power losses within the SMPSto convert primary power to secondary power wherein:

η=VSecondary*ISecondary/(VPrimary*IPrimary)=PSecondary/PPrimary.

The power converter efficiency η imposes a limit on the minimum primarypower PPrimReq required to be delivered to the SMPS primary to sustaintransmitter operations at a given ISecondary load. (see FIG. 17C). ThePPort curve of FIG. 17C is modeled by the product of ILoop*VportReq withIPrimaryCmd set to “Max” (see equation H of FIG. 12), the PPrimMax curvemodeled by IPrimaryMax*VPrimaryMaxAv with IPrimaryCmd set to “Max” (seeequation F FIG. 12) and the PPrimReq curve modeled asVPrimaryReqMin*IPrimaryReqMin with IPrimaryCmd set to “Max” (seeequation G of FIG. 12). Since, as depicted in FIG. 17C, less power Pportis available at the port and subsequently less power PPrimMax isavailable to the SMPS primary as ILoop decreases, η must be higher atlower ILoop currents to deliver the same amount of secondary powerPSecondary and current ISecondary relative to the amount deliverable athigh ILoop. As evident for the equation for VPrimReq above, higher ηresults in lower VPrimReq and, consequentially, lower VportReq. Asindicated in FIG. 17A, η is the dominating factor affecting the minimumVPrimReq and VportReq at low ILoop current.

Summarizing the above information as applied to SMPS topology selectionand design, the characteristics are important considerations. In anumber of embodiments, it is desirable that VPrimReq be as low aspossible across the operating range of ILoop to keep VPortReq as low aspossible. Lowering VPortReq allows higher loop resistance (whereinRLoop=Rmeasure+Rwire) to be accommodated for a given loop power supplyvoltage Vext. The greater latitude for increased Rwire translates intolonger possible loop distances and/or smaller wire gauges that may beaccommodated. Since VPrimReq>VPrimMin, an SMPS topology/design affordinglow VPrimMin allows VPrimReq and VportReq to be lower across theoperating range of ILoop. The impact is most significant at higher ILoopas indicated in FIG. 17A. Since VPrimReq is reduced with higher η, anSMPS topology/design affording high efficiency across the operatingrange of ILoop is desirable. Again, in a 2-wire application, this ismost impactful at low ILoop. Another consideration is the maximumvoltage the SMPS primary can sustain without damage (which is designateas the VPrimMaxLimit). This parameter, and the maximum voltage toleratedby the components connecting the SMPS primary to the transmitter portdefine the maximum voltage limit at Vport (VportMax). Since Vport risesto Vext when ILoop=0, VportMax and VPrimMaxLimit define the maximum Vextthat can be supported by the transmitter.

The buck converter, illustrated schematically in FIG. 18, regulates ahigher VPrimary to a Lower VSecondary. In this topology, VPrimary mustbe strictly higher than VSecondary so that VPrimMin>VSecondary sets thelowest possible boundary for VPrimReq and VPortReq. Buck converters canhave efficiencies in excess of 90%. This high efficiency is mostbeneficial to the 2-wire application where it results in reducedVPrimReq and VportReq for higher secondary loads ISecondary at lowILoop. Small form factor Buck controllers appropriate to the 2-wireapplication can support reasonably high maximum primary voltages(VPrimMaxLimit). The first buck controller incorporated in the anembodiment of a 2-wire system hereof supports up to 42V. In the systemanalysis set forth herein, that Buck converter is designated “FirstBuck” with VPrimMin=9.5V. The Buck converter designated “Second Buck”examines the impacts of lowering VPrimMin on the existing design to7.5V. There are limitations impacting component size needed toaccomplish such a lowering of VPrimMin which must be accommodated. Theestimates set forth herein are made with Buck converter η=90%.

The SEPIC converter illustrated in FIG. 19 is an example of a Buck-Boosttopology. Unlike the Buck converter, this topology is capable ofsupporting primary voltages VPrimary that are both above and belowVSecondary. The ability to support VPrimary<VSecondary affordsopportunity, under certain ISecondary load circumstances, tobeneficially reduce VPrimReq and VportReq. However, it is difficult toachieve high efficiency with this topology so that practicalimplementation for this application is limited to η≤75%. Because of thelower efficiency, VportReq can be higher for the SEPIC converter,despite the lower VPrimMin, at lower ILoop for large ISecondary loads.The SEPIC efficiency used in this system analysis is η=75%. As a furtherconsideration, the SEPIC converter has inherent IPrimary noisecharacteristics that are more favorable to the needs of the 2-wiretransmitter system.

To illustrate the contrast on the voltage characteristics of the SMPSand Vport with the SEPIC converter replacing the Buck converter, thecharacteristics shown for the Buck regulator in FIG. 16A are set forthin FIG. 20A with a SEPIC converter. Because of the lower VPrimReq andVportReq at high ILoop, the external load resistance with the samesecondary load as the Buck converter can be increased from 516Ω to 825Ω,which translates into an additional loop length of 9572 ft beyond the1000 ft possible with the Buck converter. However, it is important tonote that the sharp rise of VPrimReq and VportReq at low ILoop when theISecondary load is increased results in a reversal of optimal SMPSchoice for higher ISecondary.

Which SMPS topology is best suited for the 2-wire transmitter systemshereof is dependent on the 4-20 mA loop supply, loop load andISecondary. Referring to the VportReq lines in FIGS. 16A and 20A, theBuck converter can deliver the maximum secondary power for situationswhere the Vport load line intersects VPortReq at the minimal Loopcurrent. The SEPIC is more suitable in instances where Vport intersectsVportReq at the maximum ILoop current. FIG. 20B illustrates differencesin VPortReq vs ISecondary at 3.5 mA and 22 mA for the two Buckconfigurations studied herein and the SEPIC configuration. Theintersection of VportReq for all three configurations, when ILoop=3.5 mAwith a Vport of 30V, is illustrated by the dashed lines. As a result ofits lower efficiency, the VportReq for the SEPIC intersects the 30V lineat a lower secondary load current (that is, at ˜11 mA) while both Buckconverters (each having the same top intersection because each has thesame efficiency) intersect at a hither secondary load current (that is,at ˜13.2 mA). Since the VportReq at ILoop=22 mA (shown at the bottom ofthe graph of FIG. 20B) is lower than either Buck regulator, the SEPIC isclearly more optimal for ISecondary≤11 mA at VportMax=30V. ForISecondary>11 mA, the Buck topology is likely to be more optimal at aVportMax of 30V. The interplay of loop voltage, load and VportReq vsISecondary are important factors when determining the appropriate SMPStopology and the general feasibility of incorporating varioustransmitter features/loads with higher ISecondary demands. To cover thebroadest range of features, it may be preferable to partition the SMPSonto a separate printed circuit board or PCB that plugs into thetransmitter main PCB to enable a SEPIC converter to be used for low tomedium secondary loads to maximize loop distance and a Buck converter tobe used to support features drawing high secondary current.

In another alternative to, for example, buck converters and SEPICconverters, a switched capacitor converter may be used as a powerconverter in the systems hereof. As known in the art, capacitive voltageconversion may, for example, be achieved in a switched capacitor(voltage) converter by switching a capacitor periodically.

The partition labeled B in FIG. 2D includes the components andperipherals defining a core transmitter load. Generally, thosecomponents conform to the constant load category described above. Theuser input interface is separately partitioned for the convenience ofexamining different options. Likewise, sensor loads are also excludedfrom the core transmitter load.

A proposed budget allocating Isecondary current to key categories ofsecondary peripherals is enumerated in Table 1. As indicated, the totalbudget apportioned to the base secondary load in this analysis is 1.865mA.

TABLE 1 2-Wire Transmitter Standard Secondary Load Estimate Note: Doesnot include the main power supply/energy harvester overhead or UI inputoptions ISecBudget Notes uA uP PeripheralIs Ext Watchdog 35 SecondaryLin Regulation 180 and Measurement Precison Ext Reference for ADC 180Misc 50 LCD Display 100 Wired Hart 300 4_20 mA Circuit Port SecondaryCurrent 120 Sensor Port Support DC Current 100 Dynamic 485 currentallocated to sensor and is not included here uP Core Budget 800 Subtotalw/o User Interface 1865 Buttons or Sensor

Optional and/or variable features/loads in FIG. 2E are designated withthe label C. For, example, the partition labeled C1 in the lower leftcorner of FIG. 2E encompasses the portion of the total transmittersecondary load allocated to the sensor. The sensor load may, forexample, be based on an estimated worst case load presented by a certainsensor. FIGS. 21A and 21B illustrates a worst case load estimate for anelectrochemical or Echem sensor with a support sensor puck (the supportsensor puck representing a printed circuit board with the electronicsthat support operation of the sensor). The load tallied in FIGS. 21A and21B includes dynamic current consumed on the transmitter control boardfor transmitter communications. For a preliminary budget, the totaltransmitter secondary current budgeted to the sensor for the analysiswas established as 2 mA.

The partition designated C2 at the bottom of FIG. 2E captures theportion of the total transmitter secondary load allocated to a userinput interface. Three different user input interface options arediscussed herein. The secondary current estimate for each of those userinterface options is then tallied with the estimates for the basetransmitter loads and the electrochemical sensor load into a totalsecondary load estimate. Each total secondary load estimate is thenprojected to the transmitter port via the port power estimation modeldescribed in connection with FIG. 16A to determine the port requirementsto support the collective system. The projected port requirements aregenerated three times to examine the merits of three different switchmode power supplies (SMPS). These SMPS include a first Buck converterhaving a minimum primary voltage of 9.5V and 90% efficiency, a secondBuck converter having a minimum primary voltage of 7.5V with a 90%efficiency, and a SEPIC including single-ended primary convertertopology with a minimum primary voltage of 2.5V and an assumed 70%efficiency. The secondary voltage (Vsecondary) is 4.5V for allestimates. Maximum loop length estimates are generated for each SMPScase with a 250 ohm measurement resistance and a loop including 22 and18 AWG copper wire.

FIG. 22A illustrates schematically a simple magnetic user inputinterface. That type of user interface is often used in explosion-proofdesigns and includes a simple reed relay actuated by a handheld magnet.This type of interface requires the user to have access to a tool (themagnet) and tends to respond more sluggishly than the other two inputinterface embodiments discussed below. As illustrated in FIG. 22A, thisinterface draws very little secondary current (drawing switch currentonly when the reed relay is closed by the magnet) and represents thelowest power option among the three user interface options consideredherein. Combining the 10 uA allocated for this interface with thecurrents partitioned to the base transmitter and the electrochemicalsensor results in a total transmitter secondary current estimate of3.875 mA.

The port voltage required to support the secondary current load andinternal power supply primary requirements is depicted in FIG. 22B. Themaximum estimated port load and loop distance capabilities vs loopsupply voltage are illustrated in FIGS. 22C and 22D. Legacy 2-wiresystems used in connection with the ULTIMA® X and PRIMAX® I sensorsystems available from MSA Safety Incorporated of Cranberry Township,Pa. are included as benchmarks for study of transmitter replacement.Port voltages (VportReq) required at the prescribed ILoop limits (Iport)and loop length estimates for the three power converter designs (SMPS)are set forth in Table 2 of FIG. 22E.

Operational and power requirements for other features/loads forinclusion in systems hereof include: BLUETOOTH low energy (BLE),wireless HART, a combustible gas sensor, a backlit LCD, blockagedetection and other display options. There are a significant number ofvariables and factors that may affect power versus performance for suchfeatures/loads. System analysis indicated that BLE, Wireless HART and acombustible gas sensor may be supported (powered) on the 2-wiretransmitter system hereof given specific constraints on the power madeavailable at the transmitter 4-20 mA port. LCD backlighting is alsopossible. The power demands for an OLED display may, for example, falloutside the practical boundaries for a 2-wire transmitter. Blockagedetection as, for example, disclosed in U.S. Patent Publication Nos.2017/0227498 and 2017/0227499 using sensors developed for a 3-wiresystem may not be supported on the 2-wire systems hereof, but suchdetection methodologies may be supported with sensors designed to workwith the 2-wire systems hereof.

Table 3 sets forth estimates for practical power budgets to support BLE,Wireless HART and the an SGX hotplate combustible gas sensor orpellistor available from SGX Sensortech, SA of Corcelles-Coromondreche,Switzerland. Configuration and operational settings required to realizehigher performance for such load will increase the power demand. TheTotal ISecondary and loop distance estimates assume the feature/load isconnected to a transmitter with an IR touchscreen and an electrochemicalsensor.

TABLE 3 Max Loop Feature/ Max Loop Distance Feature/ Load Total DistanceUltimaX Load ISecondary ISecondary VLoop SMPS VPrimReq (ft) (ft) BLE 500 uA  4.58 mA 24 V First 12.56 8371 14785 Buck Second 11.83 11170Buck SEPIC 13.8 18000 WirelessHART  850 uA 4.925 mA 24 V First 12.588366 Buck Second 12.58 11164 Buck SEPIC 14.67 17979 WirelessHART + 1.35mA 5.425 mA 24 V First 13.64 8359 BLE Buck Second 13.64 11155 Buck SEPIC15.94 17947 SGX   5 mA 9.075 mA 24 V Second 21.38 8308 Combustible BuckSecond 21.38 11091 Buck SEPIC 25.23 — SGX   5 mA 9.075 mA 30 V First21.38 16757 23234 Combustible Buck Second 21.38 19540 Buck SEPIC 25.2325864

As set forth above, in a number of embodiments, the energy harvestingsystem hereof are used in connection with a combustible gas sensor. Thecombustible gas sensor may, for example, be a low-power combustible gassensor including a microminiature pelement system, a microelectronicmechanical systems (MEMS) or a micro-hotplate sensor as, for example,disclosed in U.S. Pat. No. 8,826,721. Although such sensors arerelatively low-power (in comparison to convention combustible gassensors including pelements), in many embodiments of systems hereof, thesensor will use most of the power harvested from the current loop.

Low thermal time constants associated with low thermal mass sensorsassist in providing quick response times, reducing the time an elementmay be unavailable for use in a detection mode and decreasing powerrequirements. In a number of embodiments, sensors hereof have a sensingelement having a thermal time constant of 8 second or less, 6 seconds orless, 1 second or less, 0.5 seconds or less or 0.250 second or less. Alow thermal mass/low thermal time constant sensor may, for example,include a MEMS pellistor as described above or a microminiature pelementof low thermal mass to provide a low thermal time constant. As usedherein the thermal time constant of an element is defined as the timerequired to change 63.2% of the total difference between its initial andfinal temperature when subjected to a step function change in drivepower, under zero power initial conditions. MEMS pellistors typicallyhave a lower thermal time constant than low-thermal-mass pelements. MEMSpellistors may, for example, have thermal time constants of 1 second orless, 0.5 seconds or less or 0.250 second or less.

In general MEMS elements for sensors hereof have a dimension less than 1mm. Such element may be manufactured via a microfabrication technique.In a number of representative embodiments, sensing elements may bemanufactured with a thick film layer suitable to cause combustion of ananalyte gas upon heating to a predetermined temperature. Sensor elementshereof may be powered to an operating temperature by resistive heatingand to detect combustible gases analytes. In a number of representativeembodiments, the thickness and diameter for a MEMS hotplate oxidativefilm is 15 microns and 650 microns, respectively.

In a number of embodiments, beads or pelements of low thermal mass andhaving low thermal constants as described above may be used as triggersensor elements as well as primary sensor element hereof.Low-thermal-mass/low-thermal-time constant pelements are, for example,discussed in U.S. Pat. No. 8,826,721, the disclosure of which isincorporated herein by reference. Such pelements may, for example, havea diameter less than 500 μm or have a volume less than a sphere having adiameter 500 μm.

Catalytic or combustible (flammable) gas sensors have been in use formany years to, for example, prevent accidents caused by the explosion ofcombustible or flammable gases. In general, combustible gas sensorsoperate by catalytic oxidation of combustible gases.

The operation of a catalytic combustible gas sensor proceeds throughelectrical detection of the heat of reaction of a combustible gas on theoxidation catalysts, usually through a resistance change. The oxidationcatalysts typically operate in a temperature above 100° C. (and moretypically about 300° C.) to catalyze combustion of an analyte (forexample, typically in the range of 300 to 700° C. temperature range formethane detection). Therefore, the sensor must sufficiently heat thesensing element through resistive heating. In a number of combustiblegas sensors, the heating and detecting element are one and the same andcomposed of a platinum alloy because of its large temperaturecoefficient of resistance and associated large signal in target/analytegas. The heating element may be a helical coil of fine wire or a planarmeander formed into a hotplate or other similar physical form. Thecatalyst being heated often is an active metal catalyst dispersed upon arefractory catalyst substrate or support structure. Usually, the activemetal is one or more noble metals such as palladium, platinum, rhodium,silver, and the like and the support structure is a refractory metaloxide including, for example, one or more oxides of aluminum, zirconium,titanium, silicon, cerium, tin, lanthanum and the like. The supportstructure may or may not have high surface area (that is, greater than75 m²/g). Precursors for the support structure and the catalytic metalmay, for example, be adhered to the heating element in one step orseparate steps using, for example, thick film or ceramic slurrytechniques. A catalytic metal salt precursor may, for example, be heatedto decompose it to the desired dispersed active metal, metal alloy,and/or metal oxide.

As illustrated in FIGS. 23A and 23B, a number of conventionalcombustible gas sensors such as illustrated sensor 10 typically includean element such as a platinum heating element wire or coil 20 encased ina refractory (for example, alumina), microminiature bead 30, which isimpregnated with a catalyst (for example, palladium or platinum) to forman active or sensing element, which is sometimes referred to as apelement 40, pellistor, detector or sensing element. A detaileddiscussion of pelements and catalytic combustible gas sensors whichinclude such pelements is found in Mosely, P. T. and Tofield, B. C.,ed., Solid State Gas Sensors, Adams Hilger Press, Bristol, England(1987). Combustible gas sensors are also discussed generally in Firth,J. G. et al., Combustion and Flame 21, 303 (1973) and in Cullis, C. F.,and Firth, J. G., Eds., Detection and Measurement of Hazardous Gases,Heinemann, Exeter, 29 (1981).

Bead 30 will react to phenomena other than catalytic oxidation that canchange its output (i.e., anything that changes the energy balance on thebead) and thereby create errors in the measurement of combustible gasconcentration. Among these phenomena are changes in ambient temperature,humidity, and pressure.

To minimize the impact of secondary effects on sensor output, the rateof oxidation of the combustible gas may, for example, be measured interms of the variation in resistance of sensing element or pelement 40relative to a reference resistance embodied in an inactive, compensatingelement or pelement 50. The two resistances may, for example, be part ofa measurement circuit such as a Wheatstone bridge circuit as illustratedin FIG. 23B. The output or the voltage developed across the bridgecircuit when a combustible gas is present provides a measure of theconcentration of the combustible gas. The characteristics ofcompensating pelement 50 are typically matched as closely as possiblewith active or sensing pelement 40. In a number of systems, compensatingpelement 50 may, however, either carry no catalyst or carry aninactivated or poisoned catalyst. In general, changes in properties ofcompensating elements caused by changing ambient conditions are used toadjust or compensate for similar changes in the sensing element.

Active or sensing pelement 40 and compensating pelement 50 can, forexample, be deployed within wells 60 a and 60 b of an explosion-proofhousing 70 and can be separated from the surrounding environment by aflashback arrestor, for example, a porous metal frit 80. Porous metalfrit 80 allows ambient gases to pass into housing 70 but preventsignition of flammable gas in the surrounding environment by the hotelements. Such catalytic gas sensors are usually mounted in instrumentswhich, in some cases, must be portable or wireless and, therefore, carrytheir own power supply. It is, therefore, desirable to minimize thepower consumption of a catalytic gas sensor.

Oxidation catalysts formed onto a helical wire heater are typicallyreferred to as pelements, while those formed onto hotplates (whethermicroelectronic mechanical systems (MEMS) hotplates or conventional,larger hotplates) are sometimes known by the substrate. Oxidativecatalysts formed on MEMS heating elements are sometimes referred toherein as MEMS pellistors. As described above, the detecting pelementsor catalytically active hotplates can be paired with a similarly sizedheater coated with materials with similar thermal conductivity as theactive catalyst but without active sites. The inactive pelement orhotplate may be used to compensate for changes in ambient temperature,relative humidity, or background thermal conductivity not associatedwith a combustible gas and are therefore often referred to ascompensators. The matched pair of detecting and compensating elementscan be assembled in a Wheatstone bridge configuration for operation andcombustible gas detection, which requires that both the detector andcompensator operate at the same elevated temperature. Thehigh-temperature operation of the catalytic sensing element requires asignificant amount of power consumption. Power consumption isparticularly a problem in the case of detecting combustible gases asdetection should be performed very often or continuously to ensure asafe environment. Portable instrument and wireless installations rely onbattery systems for power.

In several embodiments, pulse width modulation may be used to controlthe energy delivered to the hotplates. Pulse width modulation is awell-known control technique used to control the average power and/orenergy delivered to a load. In embodiments hereof, a voltage is suppliedto, for example, a MEMS hotplate or other heating element to heat thesupported catalyst to a desired temperature. Because the pellisters orpelements hereof have relatively low thermal mass, the cycle times canbe relatively short.

Heating energy (that is, heating voltage(s) or heating currents(s)) maybe periodically supplied to the heating element(s) during an “ON time”.Rest energy (that is, rest voltage(s) or a rest current(s)), which isless than the heating energy may be supplied during a “REST time”. Thetotal of the higher-energy or ON time plus the lower-energy or REST timecorrespond to a cycle time or a cycle duration. Gas concentration or theanalyte is measured during the ON time. The heating energy(voltages/currents) supplied during the ON time may be constant duringthe ON time or may be varied (for example, supplied as heatingvoltage/current plateau or as heating voltage/current ramp). The restenergy (voltages/currents) may be equal to zero, or be sufficientlylower than the heating energy so that the gas sensor does not consumeany gas or substantially any gas to be detected. Similar to the ON time,the rest energy supplied during the REST time may be constant during allthe REST time or may be varied (for example, supplied as restvoltage/current plateau or as rest voltage/current ramp). The cycle maybe repeated.

Combustible gas detectors are operated in a Wheatstone bridge as, forexample, described in connection with FIG. 23B, in constant current orconstant voltage. As described above, such sensors are powered to runthe pelements or hotplates in, for example, a temperature range of100−700° C. (and, more typically 350-600° C.) whenever the sensor isoperational. This mode of operation may be termed a “continuous” mode ofoperation. An alternate operational mode, which is particularly suitablefor-low mass pelements or MEMS hotplates/pellistors, is to quickly heatand cool the detector in a pulsed power mode. Low mass pelements are,for example, described in U.S. Pat. No. 8,826,721, the disclosure ofwhich is incorporated herein by reference. An advantage to operating inpulse mode is significantly lower power consumption as compared tocontinuous mode. Another advantage is improved span response as a resultof adsorption of excess combustible gas on the catalyst at coolertemperatures during unpowered or lower powered operation (that is,during the REST time) as compared to continuously powering the catalystat the run temperature of, for example, 350-600° C.

As used herein, the term “MEMS pellistor” refers to a sensor componentwith dimensions less than 1 mm that is manufactured via microfabricationtechniques. In a number of representative embodiments, sensing elementsformed as MEMS pellistors hereof may be manufactured with a thick filmcatalyst, powered to an operating temperature by resistive heating andare used to detect combustible gases. In a number of representativeembodiments, the thickness and diameter for a MEMS catalyst film is 15microns and 650 microns, respectively.

A representative embodiment of a MEMS sensor 100 suitable for use in anumber of studies hereof is illustrated in FIGS. 24A and 24B. The outputof MEMS pellistor 100 may, for example, be measured by connecting it astwo arms of a Wheatstone bridge as described in connection with FIG.23B. This method of measuring output is a straightforward and reliablemethod of comparing the relative change of a resistance.

MEMS hotplate sensor 100 may, for example, mounted on a printed circuitboard or PCB 200. The two resistances of the sensing element 150 and thecompensating element may, for example, be part of a measurement circuitsuch as a Wheatstone bridge circuit or a simulated Wheatstone bridgecircuit as described above. A representative example of a MEMS hotplatesensor suitable for use herein is a SGX MP7217 hotplate sensor orpellistor available from SGX Sensortech, SA of Corcelles-Coromondreche,Switzerland. Such a MEMS hotplate sensor is disclosed, for example, inU.S. Pat. No. 9,228,967, the disclosure of which is incorporated hereinby reference.

FIG. 24A illustrates a cutaway view of an embodiment of a MEMSpellistor, which includes a housing 102 having a gas inlet 110. A screenor cap 120, which may include or function as a filter 130, may, forexample, be placed in connection with inlet 110. The energy (current andvoltage) used in pellistor 100 may, for example, be sufficiently low toprovide intrinsic safety such that a flashback arrestor, as known in thecombustible gas detector arts, may not be necessary. Flashback arrestors(for example, porous frits) allow ambient gases to pass into a housingbut prevent ignition of combustible/flammable gas in the surroundingenvironment by hot elements within the housing. One or more heatingelements or hotplates 140 may be used to heat an oxidative catalystlayer 150 to operating temperature. A MEMS compensating element orcompensator may be included within MEMS pellistor 100. As describedabove, MEMS compensator may include an inactive layer 150′ which may beheated by one or more heating elements or hotplates 140′. Alternativelylayer 150′ may include an active catalyst and be operated at asufficiently low temperature to prevent oxidation of combustible gas.MEMS pellistor 100 is mounted on a PCB 200 as described above. In anumber of studies, a SGX MP7217 pellistor available from SGX Sensortech,SA of Corcelles-Coromondreche, Switzerland was used in the studiedsystems hereof.

Catalytic combustible sensors offer an end user a sensing solution whichcan detect a vast variety of flammable gasses. While other technologies(such as nondispersive infrared or NDIR sensors) have achieved lowerpower levels, they are not as broad in sensing capability as a catalyticcombustible gas sensor. NDIR sensors cannot, for example, detecthydrogen, a key industrial hazard. The use of catalytic sensors has notyet been offered in a two-wire configuration because of their high powerconsumption. Standard or conventional catalytic permanent sensors havepower requirements of 1000 mW or more. A typical portable systemconsumes approximately 250 mW. Sensors taking advantage of advancementsin miniaturization using MEMS, miniature coils and microhotplatetechnologies, while operated in a continuous power mode have powerrequirements which are still above 100 mW. A catalytic combustiblesensor solution which operates within the constraints of a two-wiresystem would represent a significant advancement in the art.

A pulse mode operation of a combustible gas sensor includinglow-thermal-mass element in which the combustible sensor is operated fora period of time short enough to measure the target gas and thendiscontinue its use long enough to store the power needed for the nextcycle provides a solution. This operational mode is sufficientlyflexible to adapt to changing measurement requirements, efficient enoughto store the required power without exceeding the 4 mA operatinglimitations of a 4-20 mA, and electrically quiet enough not to disruptthe system integrity.

A commercially available catalytic microhotplate device (SGX MP-7217)can reach operating temperatures in 30-50 mS and stabilize in 100 mS.During this period of operation, the device would require 120 mW. Whenconsidering a typical response time requirement of 10 seconds, adesirable operating interval may, for example, be 1 second. Therefore,if the remainder of the system is active while the catalytic combustiblegas sensor is in an off state, the system needs to store enough energyfor the next cycle and complete any other tasks. For example, if thesystem uses 20 mW while the sensors is off, the average power would be[(120 mW*0.100)+(20 mW*0.900)]=30 mW and thus meet the system needs.While this is overly simplified one skilled in the art will appreciatehow the goal of powering a catalytic combustible sensor within theconstraints of power harvested from a two-wire system.

A collection of various methane responses is depicted in FIG. 25 for acommercially available microhotplate device (SGX MP-7217). At 100 ms,the sensor is able to discern that a gas is present, and an accurateanalysis can be accomplished at 250 ms. From FIG. 25, one can concludethat the sensor would be consuming an average power of 30 mW at a 250 mson-time interval, meeting the system requirements discussed above.Further, when the gas concentration reaches 50% as shown in Table 4 ofFIG. 26, the SGX MP-7127 sensor could remain on all the time if sodesired. Similar data is provided for an MSA XCell® combustible gassensor available from MSA of Cranberry Township, Pa. in Table 5 of FIG.27. In general, when there is gas present, the loop current will behigher and more power is available. One may, for example, shorten theduty cycle when gas is present because of in the increase in poweravailable. One could also deploy a mixture of devices that are bothlower and higher power and switch in alternate technologies as morepower becomes available.

As set forth above, the devices, systems and method methodology hereofmay be used in connection with any low-power combustible gas sensors.Typically, such sensors include low-thermal-mass elements such asmicrohotplates or microminiature coil pelements. One may merge a varietyof power savings techniques such as pulse width modulation or the splitbridge operation as in the apparatus used in U.S. Pat. No. 8,826,721,wherein the elements are operated independently. In a number ofembodiments, the combustible gas sensor is operated so that it can be inan off state long enough to store the power required when the sensorreturns to an on state. Continuous operational modes, as describedabove, may also be possible.

Control modes and electronics for the operation of combustible gassensor elements suitable for use herein are, for example, disclosed inU.S. patent application Ser. Nos. 15/597,933 and 15/597,859, and U.S.Pat. Nos. 8,826,721, 4,533,520 and 5,780,715, the disclosures of whichare incorporated herein by reference. Such methodologies and circuitsmay be readily adapted for use herein.

The foregoing description and accompanying drawings set forth a numberof representative embodiments at the present time. Variousmodifications, additions and alternative designs will, of course, becomeapparent to those skilled in the art in light of the foregoing teachingswithout departing from the scope hereof, which is indicated by thefollowing claims rather than by the foregoing description. All changesand variations that fall within the meaning and range of equivalency ofthe claims are to be embraced within their scope.

What is claimed is:
 1. A system comprising a two-conductor loop in electrical connection with a power source and with a loop current controller, the loop current controller comprising a first port having a first terminal and a second terminal to connect to respective conductors of the two-conductor loop and a second port in operative connection with a sensor, the loop current controller controlling a current in the two-conductor loop to be equal to a current signal to transmit the current signal to a receiver connected to the two-conductor loop, the current in the two-conductor loop being controlled by the loop current controller to be proportional to a signal output from the sensor, the system further comprising energy harvesting circuitry in electrical connection with the two-conductor loop, the energy harvesting circuitry comprising a second current controller in parallel electrical connection with the loop current controller and a power converter in electrical connection with the second current controller, the second current controller controlling a portion of current drawn from the two-conductor loop and delivered to the power converter from an output port of the second current controller, the portion of the current drawn from the two-conductor loop being less than the current signal into an input port thereof, the power converter converting a primary current and a primary voltage to a secondary current and a predetermined secondary voltage to be supplied to at least one load device, the portion of the current drawn from the two-conductor loop being returned to the loop current controller from the energy harvesting circuit, wherein noise in the portion of the current drawn from the two-conductor loop by the second current controller is controlled by the second current controller to be below a predetermined threshold so that the loop current controller controls noise in the current in the two-conductor loop to be less than a current corresponding to the resolution of the sensor.
 2. The system of claim 1 wherein noise in the portion of the current drawn from the two-conductor loop to the secondary current controller is controlled so that the loop current controller controls noise in the current in the two-conductor loop to be less than a current corresponding to one half the resolution of the sensor.
 3. The system of claim 2 wherein the second current controller has a secondary bandwidth and the loop current controller has a first bandwidth, the secondary bandwidth being greater than the first bandwidth.
 4. The system of claim 3 wherein the secondary bandwidth is sufficient to resist variation in the portion of current drawn from the two-conductor loop in response to variation in the primary current and the primary voltage arising in the energy harvesting circuit.
 5. The system of claim 4 wherein the second current controller restricts noise in the current drawn from the two-conductor loop such that the first bandwidth of the loop current controller is sufficient to restrict noise in the current signal to be within one half of the resolution of the sensor.
 6. The system of claim 4 further comprising a processor system in operative connection with the sensor to receive an analog signal from the sensor, the processor system converting the analog signal to a digital signal to be delivered to the loop current controller to establish a first setpoint equivalent to the current signal.
 7. The system of claim 6 wherein the processor system is in operative connection with the second current controller to set a second setpoint equal the portion of the current drawn from the two-conductor loop by the second current controller based upon data feedback to the processor system indicating a status of power conversion.
 8. The system of claim 7 wherein the loop current controller draws a current I1 from the two-conductor loop such that I1 plus the returned portion of the current drawn from the two-conductor loop by the second current controller is equal to the first setpoint, wherein the second setpoint is always less than the first setpoint.
 9. The system of claim 8 wherein the primary current is controlled to maintain the power input to the power converter in a predetermined manner based upon data feedback to the processor system indicating a status of power conversion.
 10. The system of claim 7 wherein the energy harvesting system further comprises a third current controller having an input port in electrical connection with an output of the power converter, the third current controller determining the secondary current drawn through the power converter.
 11. The system of claim 10 wherein the energy harvesting system further comprises at least one energy storage component in electrical connection with an output of the third current controller and in electrical connection with the at least one load device.
 12. The system of claim 11 wherein the at least one energy storage component comprises at least one high-capacity capacitor.
 13. The system of claim 12 wherein energy is transferred to the at least one load device from the at least one high-capacity capacitor if an instantaneous power requirement of the at least one load device exceeds an instantaneous power available from the power converter.
 14. The system of claim 12 wherein energy is transferred to the at least one load device from the at least one high-capacity capacitor if the primary voltage is below a predetermined threshold.
 15. The system of claim 13 wherein energy is transferred to the at least one high-capacity capacitor if the instantaneous power required by the at least one load device is less than the instantaneous power energy available from the power converter.
 16. The system of claim 14 wherein the processor system is in operative connection with the third current controller, determines a third setpoint equivalent to the secondary current to be drawn through the power converter and transmits the third setpoint to the third current controller.
 17. The system of claim 16 wherein the processor system further controls energy transfer from the at least one high-capacity capacitor.
 18. The system of claim 16 wherein the third setpoint is determined by the processor system on the basis of at least one of a power available to the power converter and a predetermined operating range of the primary current and the primary voltage.
 19. The system of claim 16 wherein the second setpoint and the third setpoint are determined to maintain each of the primary current and the primary voltage in a predetermined range.
 20. The system of claim 26 wherein the second setpoint and the third setpoint are determined to restrict noise in the primary current arising from the power converter.
 21. The system of claim 16 wherein the at least one load device is a combustible gas sensor.
 22. The system of claim 21 wherein the combustible gas sensor is the sensor from which the signal is output and comprises a first element, the first element comprising a first electric heating element, a first support structure on the first electric heating element and a first catalyst supported on the first support structure, the combustible gas sensor further comprising electronic circuitry in electrical connection with the energy harvesting system and with the first element, the electronic circuitry providing energy to the first element to heat the first element to at least a first temperature at which the first catalyst catalyzed combustion of an analyte gas and the electronic circuitry applies energy to the first element in a pulsed manner.
 23. The system of claim 22 comprising a plurality of load devices comprising the combustible gas sensor in operative connection with the power conversion system, wherein a capacity of the at least one high-capacity capacitor is determined based upon a predicted load profile of the plurality of load devices.
 24. The system of claim 23 wherein the processor system controls application of power to at least one of the plurality of load devices based upon a predetermined rule set.
 25. The system of claim 4 wherein the energy harvesting system further comprises at least one energy storage component in electrical connection with an output of the power converter and in electrical connection with the at least one load device.
 26. The system of claim 27 wherein the at least one energy storage component comprises at least one high-capacity capacitor.
 27. The system of claim 28 wherein energy is transferred to the at least one load device from the at least one high-capacity capacitor if an instantaneous power requirement of the at least one load device exceeds an instantaneous power available from the power converter.
 28. The system of claim 1 wherein the power converter comprises a DC-DC switched-mode power supply to provide a predetermined secondary voltage.
 29. The system of claim 1 wherein the power converter comprises a buck converter, a sepic converter or a switched capacitor converter to provide a predetermined secondary voltage.
 30. The system of claim 1 wherein an impedance looking into the output of the second current controller is greater than an impedance looking into a bypass capacitance in electrical connection of the output of the second current controller and with the input of the power converter.
 31. The system of claim 20 the impedance looking into the output of the second current controller is sufficiently greater than the impedance of the input looking into the bypass capacitance so that harmonics above the second bandwidth are drawn from the bypass capacitance.
 32. A method of harvesting energy from a two-conductor loop in electrical connection with a power source and with a loop current controller, the loop current controller comprising a first port having a first terminal and a second terminal to connect to respective conductors of the two-conductor loop and a second port in operative connection with a sensor, the loop current controller controlling a current in the two-conductor loop to be equal to a current signal to transmit the current signal to a receiver connected to the two-conductor loop, the current in the two-conductor loop being controlled by the loop current controller to be proportional to a signal output from the sensor, comprising: electrically connecting an energy harvesting circuit to the two-conductor loop, the energy harvesting circuit comprising a second current controller connected in parallel electrical connection with the loop current controller and a power converter in electrical connection with an output of the second current controller, the second current controller controlling a portion of current drawn from the two-conductor loop and delivered to the power converter from an output port of the second current controller, the portion of the current drawn from the two-conductor loop being less than the current signal into an input port thereof, converting the primary current and a primary voltage to a secondary current and a predetermined secondary voltage to be supplied to at least one load device via the power converter, and returning the portion of the current drawn from the two-conductor loop to the loop current controller from the energy harvesting circuit, wherein noise in the portion of the current drawn from the two-conductor loop by the second current controller is controlled by the second current controller to be below a predetermined threshold so that the loop current controller controls noise in the current in the two-conductor loop to be less than a current corresponding to the resolution of the sensor.
 33. A system comprising a two-conductor loop in electrical connection with a power source and with a loop current controller, the loop current controller comprising a first port having a first terminal and a second terminal to connect to respective conductors of the two-conductor loop and a second port in operative connection with a combustible gas sensor, the loop current controller controlling a current in the two-conductor loop to be equal to a current signal to transmit the current signal to a receiver connected to the two-conductor loop, the current in the two-conductor loop being controlled by the loop current controller to be proportional to a signal output from the combustible gas sensor, the system further comprising energy harvesting circuitry in electrical connection with the two-conductor loop, the energy harvesting circuitry comprising a second current controller in parallel electrical connection with the loop current controller and a power converter in electrical connection with the second current controller, the second current controller drawing a portion of current from the two-conductor loop which is less than the current signal into an input port thereof, the second current controller controlling a portion of current drawn from the two-conductor loop and delivered to the power converter from an output port of the second current controller, the portion of the current drawn from the two-conductor loop being less than the current signal into an input port thereof, the power converter converting a primary current and a primary voltage to a secondary current and a predetermined secondary voltage to be supplied to at least one load device, the portion of the current drawn from the two-conductor loop being returned to the loop current controller from the energy harvesting circuit, wherein noise in the portion of the current drawn from the two-conductor loop by the second current controller is controlled by the second current controller to be below a predetermined threshold so that the loop current controller controls noise in the current in the two-conductor loop to be less than a current corresponding to the resolution of the combustible gas sensor. 